[tmp]$ mkdir main bare sub
[tmp]$ git init --bare bare
Initialized empty Git repository in /tmp/tmp/bare/
[tmp]$ cd main
[main]$ git init .
Initialized empty Git repository in /tmp/tmp/main/.git/
[main]$ git config user.name main; git config user.email main
[main]$ echo secret > secret
[main]$ mkdir lib
[main]$ echo from main > lib/libfile
[main]$ git add .
[main]$ git commit -m initial
[master (root-commit) 1fadd18] initial
 2 files changed, 2 insertions(+)
 create mode 100644 lib/libfile
 create mode 100644 secret
[main]$ git subtree split --rejoin -P lib/ -b subbranch master
Merge made by the 'ours' strategy.
Created branch 'subbranch'
30707246d9fe28bd4ea0227823b1c2464198a7fb
[main]$ git push ../bare subbranch:master
Counting objects: 3, done.
Writing objects: 100% (3/3), 200 bytes | 0 bytes/s, done.
Total 3 (delta 0), reused 0 (delta 0)
To ../bare
 * [new branch]      subbranch -> master
[main]$ pushd ../sub
/tmp/tmp/sub /tmp/tmp/main
[sub]$ git clone ../bare .
Cloning into '.'...
done.
[sub]$ git config user.name sub; git config user.email sub
[sub]$ echo from sub >> libfile
[sub]$ cat libfile
from main
from sub
[sub]$ git commit -am "from sub"
[master d521e37] from sub
 1 file changed, 1 insertion(+)
[sub]$ git push
Counting objects: 3, done.
Writing objects: 100% (3/3), 234 bytes | 0 bytes/s, done.
Total 3 (delta 0), reused 0 (delta 0)
To /tmp/tmp/sub/../bare
   3070724..d521e37  master -> master
[sub]$ popd
/tmp/tmp/main
[main]$ git subtree pull -P lib/ ../bare  master
remote: Counting objects: 3, done.        
remote: Total 3 (delta 0), reused 0 (delta 0)        
Unpacking objects: 100% (3/3), done.
From ../bare
 * branch            master     -> FETCH_HEAD
error: Terminal is dumb, but EDITOR unset
Not committing merge; use 'git commit' to complete the merge.
[main]$ git commit -m merge
[master fbd74e8] merge
[main]$ git --no-pager log --stat
commit fbd74e87493650049eed0ff91be444c33a00be37
Merge: 638273f d521e37
Author: main <main>
Date:   Sun Aug 12 09:51:44 2018 +0800

    merge

commit d521e376da6c06af0b8b0048d06038591ba953de
Author: sub <sub>
Date:   Sun Aug 12 09:51:34 2018 +0800

    from sub

 libfile | 1 +
 1 file changed, 1 insertion(+)

commit 638273fb345cb5d924f50d86498d71db20f8cd1f
Merge: 1fadd18 3070724
Author: main <main>
Date:   Sun Aug 12 09:51:15 2018 +0800

    Split 'lib/' into commit '30707246d9fe28bd4ea0227823b1c2464198a7fb'
    
    git-subtree-dir: lib
    git-subtree-mainline: 1fadd1800d4323a1ee2afda317f1422d1ce88786
    git-subtree-split: 30707246d9fe28bd4ea0227823b1c2464198a7fb

commit 30707246d9fe28bd4ea0227823b1c2464198a7fb
Author: main <main>
Date:   Sun Aug 12 09:51:12 2018 +0800

    initial

 libfile | 1 +
 1 file changed, 1 insertion(+)

commit 1fadd1800d4323a1ee2afda317f1422d1ce88786
Author: main <main>
Date:   Sun Aug 12 09:51:12 2018 +0800

    initial

 lib/libfile | 1 +
 secret      | 1 +
 2 files changed, 2 insertions(+)
[main]$ cd ../bare
[bare]$ ls
branches  config  description  HEAD  hooks  info  objects  refs
[bare]$ cd objects/
[objects]$ find -type f | tr -d ./ | xargs -L1 -i bash -c 'echo contents of {} ---------------; git cat-file -p {}'
contents of d521e376da6c06af0b8b0048d06038591ba953de ---------------
tree fd300cd47abfb8776165b929faed376ea3205306
parent 30707246d9fe28bd4ea0227823b1c2464198a7fb
author sub <sub> 1534038694 +0800
committer sub <sub> 1534038694 +0800

from sub
contents of 26f343c083952217e74b016dda8c88e11cf5b394 ---------------
from main
contents of 30707246d9fe28bd4ea0227823b1c2464198a7fb ---------------
tree 7696b154e1692d288941d66c4cff126d57d847a6
author main <main> 1534038672 +0800
committer main <main> 1534038672 +0800

initial
contents of d2292eabd746fb92dd0f20f51d8fcced705ca5aa ---------------
from main
from sub
contents of 7696b154e1692d288941d66c4cff126d57d847a6 ---------------
100644 blob 26f343c083952217e74b016dda8c88e11cf5b394    libfile
contents of fd300cd47abfb8776165b929faed376ea3205306 ---------------
100644 blob d2292eabd746fb92dd0f20f51d8fcced705ca5aa    libfile