如果你的项目中有很多第三方的lib，你希望使用它，并且也希望可能对该lib做修改并且贡献到原始的项目中去，或者你的项目希望模块化，分为几个repo单独维护，那么git subtree就是一个选择。git subtree管理的子项目在父项目中作为一个完整的代码copy存在，并不包含历史信息。综合考虑git subtree和git submodule的优缺点，一个可行的管理策略是：使用git subtree对父项目（在该父项目中可以临时将子项目文件夹加入到tracking中）做split,将需要单独存在的目录形成一个子项目，并且push出来。随后再父项目的开发中，将子项目目录加入git ignore中，不做版本跟踪，子项目目录以另一个git repo存在（git clone子项目）。所有本地副项目中的修改如果需要分享的话，只在子项目repo中提交并且push，其他成员只需pull这个子项目repo。这个策略是一个折中。
git subtree是附属于git的附加工具，默认情况下并不会安装。https://github.com/git/git/tree/master/contrib/subtree。注意新版本的git中已经默认包含了subtree这个子命令
#git clone https://github.com/git
# cd git# make prefix=/usr/local/git install
# echo "export PATH=$PATH:/usr/local/git/bin" >> /etc/bashrc
# source /etc/bashrc
# cd contrib/subtree
# makeprefix=/usr/local/git/bin
# makeprefix=/usr/local/git/bin install
# makeprefix=/usr/local/git/bin install-doc
# cp git-subtree/usr/local/git/bin :一旦将git-bustree copy到git/bin目录下，执行git subtree 命令时，git就可以查找并且调用到这个subtree子命令了！
就ok了！在本文中，我们假设我们工作在subtree-p这个项目中（放在github上的一个repo），而在该项目中，我们需要subtree-sub这个子模块（或者说是Lib）.
当前我们的subtree-p项目中，没有引入任何subtree-sub这个模块，现在我们通过以下命令来实现：
(master)$ git remote add subtree-sub-remote https://github.com/cnweibo/subtree-sub.git
(master)$ git remote -v
origin https://github.com/cnweibo/subtree-p.git (fetch)
origin https://github.com/cnweibo/subtree-p.git (push)
subtree-sub-remote https://github.com/cnweibo/subtree-sub.git (fetch)
subtree-sub-remote https://github.com/cnweibo/subtree-sub.git (push)
(master)$ pwd
/home/cabox/workspace/subtreetest/subtree-p
(master)$ ls
README.md
(master)$ git subtree add --prefix=mysubtree subtree-sub-remote master
git fetch subtree-sub-remote master
warning: no common commits
remote: Counting objects: , done.
remote: Total (delta ), reused (delta ), pack-reused
Unpacking objects: % (/), done.
From https://github.com/cnweibo/subtree-sub
* branch master -> FETCH_HEAD
* [new branch] master -> subtree-sub-remote/master
Added dir 'mysubtree'
(master)$ ls
README.md mysubtree
(master)$ ls -la mysubtree/
total
drwxrwxr-x cabox cabox May : .
drwxrwxr-x cabox cabox May : ..
-rw-rw-r-- cabox cabox May : README.md
上面的一系列命令中，首先在subtree-p项目目录中引入了subtree-sub这个子项目，它存放在目录mysubtree中，和我们的subtree-sub-remote这个remote相对应。这时，subtree-p项目由于增加了新的目录（subtree-sub子项目repo），因此需要commit和push.
注意：在mysubtree这个子目录中是没有任何.git文件的，它实际上是subtree-sub这个子项目repo的纯代码copy！！ git subtree add命令本身不会像git submodule add一样添加任何meta data@！！
(master)$ git s
On branch master
Your branch is ahead of 'origin/master' by commits.
(use "git push" to publish your local commits)
nothing to commit, working directory clean
(master)$ git push
Username for 'https://github.com': kidsit
Password for 'https://kidsit@github.com':
Counting objects: , done.
Delta compression using up to threads.
Compressing objects: % (/), done.
Writing objects: % (/), bytes | bytes/s, done.
Total (delta ), reused (delta )
To https://github.com/cnweibo/subtree-p.git
67da25d..c1e6018 master -> master
这时，我们查看在github上的subtree-p repo，你就可以看到这个父项目已经引入了mysubtree（代表了subtree-sub repo！）
现在我们在parent项目中对子项目做修改并且commit, push：
(master)$ echo "kidsit add subfile1 within parent" >subfile1
(master)*$ git s
On branch master
Your branch is up-to-date with 'origin/master'.
Untracked files:
(use "git add <file>..." to include in what will be committed) subfile1 nothing added to commit but untracked files present (use "git add" to track)
(master)*$ git a .
(master)*$ git ci "kidsit add subfile1 within parent"
[master ed30a68] kidsit add subfile1 within parent
file changed, insertion(+)
create mode mysubtree/subfile1
(master)$ git s
On branch master
Your branch is ahead of 'origin/master' by commit.
(use "git push" to publish your local commits)
nothing to commit, working directory clean
(master)$ git push
Username for 'https://github.com': kidsit
Password for 'https://kidsit@github.com':
Counting objects: , done.
Delta compression using up to threads.
Compressing objects: % (/), done.
Writing objects: % (/), bytes | bytes/s, done.
Total (delta ), reused (delta )
To https://github.com/cnweibo/subtree-p.git
c1e6018..ed30a68 master -> master
(master)$
这时我们再查看github上的subtree-p我们就能看到我们对子项目的新的改动了：
我们来继续模拟代码开发过程：修改subtree-p和subtree-sub后，统一一次性做一个提交：
(master)$ echo "kidsit change README for p" >README.md
(master)*$ echo "kidsit add new subfileNew2 " >mysubtree/subfileNew2
(master)*$ git s
On branch master
Your branch is up-to-date with 'origin/master'.
Changes not staged for commit:
(use "git add <file>..." to update what will be committed)
(use "git checkout -- <file>..." to discard changes in working directory) modified: README.md Untracked files:
(use "git add <file>..." to include in what will be committed) mysubtree/subfileNew2 no changes added to commit (use "git add" and/or "git commit -a")
(master)*$ git a .
(master)*$ git ci "kidsit change p and s in ONE commit"
[master a27d335] kidsit change p and s in ONE commit
files changed, insertions(+), deletions(-)
create mode mysubtree/subfileNew2
(master)$ git push
Username for 'https://github.com': kidsit
Password for 'https://kidsit@github.com':
Counting objects: , done.
Delta compression using up to threads.
Compressing objects: % (/), done.
Writing objects: % (/), bytes | bytes/s, done.
Total (delta ), reused (delta )
To https://github.com/cnweibo/subtree-p.git
ed30a68..a27d335 master -> master
(master)$
这时，我们要注意，我们再subtree-p项目中对父项目和子项目分别都做了修改，但是子项目本身的repo还没有做过任何更新，它根本不知道你曾经做过修改，这时，如果你希望将你对子项目的修改贡献出来，可以执行git subtree push命令
(master)$ git push
Username for 'https://github.com': kidsit
Password for 'https://kidsit@github.com':
Counting objects: , done.
Delta compression using up to threads.
Compressing objects: % (/), done.
Writing objects: % (/), bytes | bytes/s, done.
Total (delta ), reused (delta )
To https://github.com/cnweibo/subtree-p.git
ed30a68..a27d335 master -> master
(master)$ git subtree push --prefix=mysubtree/ subtree-sub-remote master
git push using: subtree-sub-remote master
Username for 'https://github.com': kidsit
Password for 'https://kidsit@github.com':
Counting objects: , done.
Delta compression using up to threads.
Compressing objects: % (/), done.
Writing objects: % (/), bytes | bytes/s, done.
Total (delta ), reused (delta )
To https://github.com/cnweibo/subtree-sub.git
9a0ef93..001c5d8 001c5d86c4da43ab676aaeb49763d5353353159f -> master
上面我们分别对subtree-p这个父项目和subtree-sub这个子项目分别作了push操作分享（一般来说对子项目push是非常少见的动作，因为我们更多的是拿别人的lib来用而不是开发），这时如果我们去看github上的两个项目repo的commit历史，你会发现两个repo上都有了同一个commit！！
正因为如此，我们最好是将父项目和子项目分别做commit，不要混淆在一起！这样做的好处是git subtree push时，git会自动将对父项目和子项目的commit区分清楚的，对父项目的修改不会出现在子项目repo修改历史中！
现在，我们假设另外一个人对subtree-sub这个repo独立做了新的开发，有了一个commit,但是我们的subtree-p项目实际上对此毫不知情，我们怎么能够获取别人对我们用到的子项目lib来更新呢？
在父项目目录中执行git subtree pull命令：
(master)$ git subtree pull --prefix=mysubtree -squash subtree-sub-remote master
You need to run this command from the toplevel of the working tree.
(master)$ kidsit
-bash: kidsit: command not found
(master)$ pwd
/home/cabox/workspace/subtreetest/subtree-p/mysubtree
(master)$ cd ..
(master)$ ls
README.md mysubtree
(master)$ pwd
/home/cabox/workspace/subtreetest/subtree-p
(master)$ git subtree pull --prefix=mysubtree subtree-sub-remote master
remote: Counting objects: , done.
remote: Compressing objects: % (/), done.
remote: Total (delta ), reused (delta ), pack-reused
Unpacking objects: % (/), done.
From https://github.com/cnweibo/subtree-sub
* branch master -> FETCH_HEAD
001c5d8..55689fc master -> subtree-sub-remote/master
Merge made by the 'recursive' strategy.
mysubtree/standardaloneChangeInsubtreesubRepo | +
file changed, insertion(+)
create mode mysubtree/standardaloneChangeInsubtreesubRepo
(master)$ git lg
* 4d2a47a (HEAD -> master) Merge commit '55689fc679ec0eeaf0a7853cd348309dad99fb96' from cnweibo update as
standard dev
|\
| * 55689fc (subtree-sub-remote/master) new content as a standard repo dev env for subtree-sub by cnweibo
| * 001c5d8 kidsit change p and s in ONE commit
| * ef33027 kidsit add subfile1 within parent
* | a27d335 (origin/master, origin/HEAD) kidsit change p and s in ONE commit
* | ed30a68 kidsit add subfile1 within parent
* | c1e6018 Add 'mysubtree/' from commit '9a0ef93b82b73fee89dd3d5123d441680c2a7e40'
|\ \
| |/
| * 9a0ef93 Initial commit
* 67da25d p
* f05da04 Create README.md
(master)$ ls
.git/ README.md mysubtree/
(master)$ ls mysubtree/
README.md subfile1
standardaloneChangeInsubtreesubRepo subfileNew2
(master)$ ls mysubtree/
README.md standardaloneChangeInsubtreesubRepo subfile1 subfileNew2
(master)$ git push
Username for 'https://github.com': kidsit
Password for 'https://kidsit@github.com':
Counting objects: , done.
Delta compression using up to threads.
Compressing objects: % (/), done.
Writing objects: % (/), 1.06 KiB | bytes/s, done.
Total (delta ), reused (delta )
To https://github.com/cnweibo/subtree-p.git
a27d335..4d2a47a master -> master
(master)$
上述命令主要是在subtree-p项目中pull别的独立项目对subtree-sub子模块的贡献，随后git push到主项目的repo中独立引用那一份修改。
接下来，我们来看新的项目成员进到我们的subtree-p项目中，他又该如何呢？
他首先需要做的是git clone yourParentRep, 随后git rm -r subModule, git add ,git commit.这时他的working directory中不再含有subModule的代码，他需要做的是手工创建git remote add --prefix=submodDirectory submodRemote master/v1.0,注意：在这里可以指定一个tag来选择相应的历史版本！！！ git subtree add submodRemote xxxx.git master获取subrep代码到你的submodDirectory目录，这样他自己clone的repo就有了和submodRepo的联系了，他也可以对父项目和子项目来做贡献了。
请参考： https://lostechies.com/johnteague/2014/04/04/using-git-subtrees-to-split-a-repository/
git subtree方式管理的parent项目最终是包含所有子模块的完整代码的（可以不含子模块的历史），同时支持对子项目的contribute。
在一个前台可能被share，后台可能从一个换成另外一个，这样的场景就比较适合使用subtree来管理。
接下来我们再看另外一种情况： 在一个项目中，当我们完整做完稳定版本后，我们希望将public前端拿出来作为一个独立的repo来管理，这样前端和后端完全可以独立开发，要实现这个，就需要使用git subtree split命令了：
(master)$ ls
README.md mysubtree
(master)$ git subtree split --prefix=mysubtree -b splittedbranch
Created branch 'splittedbranch'
55689fc679ec0eeaf0a7853cd348309dad99fb96
(master)$ git s
On branch master
Your branch is up-to-date with 'origin/master'.
nothing to commit, working directory clean
(master)$ git branch -a
* master
splittedbranch
remotes/origin/HEAD -> origin/master
remotes/origin/master
(master)$ ls
README.md mysubtree
(master)$ git remote add splittedRemote https://github.com/cnweibo/splitted.git
(master)$ git checkout splittedbranch
Switched to branch 'splittedbranch'
(splittedbranch)$ ls
README.md standardaloneChangeInsubtreesubRepo subfile1 subfileNew2
(splittedbranch)$ pwd
/home/cabox/workspace/subtreesplit/superrepo
(splittedbranch)$ ls
README.md standardaloneChangeInsubtreesubRepo subfile1 subfileNew2
(splittedbranch)$ git remote -v
origin https://github.com/cnweibo/subtree-p.git (fetch)
origin https://github.com/cnweibo/subtree-p.git (push)
splittedRemote https://github.com/cnweibo/splitted.git (fetch)
splittedRemote https://github.com/cnweibo/splitted.git (push)
(splittedbranch)$ git push splittedRemote splittedbranch:master
Username for 'https://github.com': kidsit
Password for 'https://kidsit@github.com':
Counting objects: , done.
Delta compression using up to threads.
Compressing objects: % (/), done.
Writing objects: % (/), 1.43 KiB | bytes/s, done.
Total (delta ), reused (delta )
To https://github.com/cnweibo/splitted.git
2d00c85..703b820 splittedbranch -> master
上面的一串命令将parent项目的mysubtree目录被splitted到splittedbranch上（当你checkout这个splittedbranch时，你看到的就只有这个目录的文件了！），随后将该branch上的内容push到remote中，这就形成了独立的repo了。随后你要做的是在父项目中，将该子文件夹删除，通过git subtree来引用这个共享的repo
http://blog.nwcadence.com/git-subtrees/
注意：git filter-branch 也可以实现类似的git subtree split的功能，将一个repo的部分目录分割为一个新的repo.
具体可参考： https://help.github.com/articles/splitting-a-subfolder-out-into-a-new-repository/
另外一篇文章： http://log.pardus.de/2012/08/modular-git-with-git-subtree.html 可以参考更好的思路