git clone git@github.com:XXX/repo1.git repo1
cd repo1

git remote add git@github.com:XXX/repo2.git repo2

git checkout -b repo1_work
git branch remote/repo2/master repo2_work

git merge -Xtheirs --allow-unrelated-histories repo2_work

rm -rf repo1 repo2

mkdir repo1
( cd repo1
  git init
  echo hoge > hoge
  git add hoge
  git commit -m "Initial"
  echo fuga >> hoge
  git add hoge
  git commit -m "Append fuga" )

git clone repo1 repo2
( cd repo2
  rm -rf .git
  git init
  git add -A
  git commit -m "Re-initialize"
  echo piyo >> hoge
  git add hoge
  git commit -m "Append piyo" )

( cd repo2
  git remote add repo1 ../repo1
  git fetch -p repo1 )

( cd repo1
  git log --graph > ../repo1-before.txt
)

( cd repo2
  git log --graph > ../repo2-before.txt
)

* commit 87ee5eb689e4aa33c4b6816c501d89ee154658ea
| Author: Yuki INOUE <my.mail@domain.com>
| Date:   Fri Nov 3 11:47:18 2017 +0000
|
|     Append fuga
|
* commit 7706f3beea43965c3dd92a9fcc914bbe4f91efbf
  Author: Yuki INOUE <my.mail@domain.com>
  Date:   Fri Nov 3 11:47:18 2017 +0000

      Initial

* commit 59a6096bdad4da18b1055fc66f49e4d54bc05be4
| Author: Yuki INOUE <my.mail@domain.com>
| Date:   Fri Nov 3 11:47:18 2017 +0000
|
|     Append piyo
|
* commit 3fd04fd77a62b2f5471902eb0f020ade791c598c
  Author: Yuki INOUE <my.mail@domain.com>
  Date:   Fri Nov 3 11:47:18 2017 +0000

      Re-initialize

repo1_end=$(
    cd repo1
    git rev-parse HEAD )

repo2_init=$(
    cd repo2
    git rev-list --max-parents=0 HEAD )

( cd repo2
  git replace $repo2_init $repo1_end
)

# repo2 では、一時的に、 `.git/info/replace` が書き換えられ、
# repo2_init が repo1_end で解決されている。

( cd repo2
  git log --graph > ../repo2_replaced.txt # 確認する。
)

* commit 59a6096bdad4da18b1055fc66f49e4d54bc05be4
| Author: Yuki INOUE <my.mail@domain.com>
| Date:   Fri Nov 3 11:47:18 2017 +0000
|
|     Append piyo
|
* commit 3fd04fd77a62b2f5471902eb0f020ade791c598c
| Author: Yuki INOUE <my.mail@domain.com>
| Date:   Fri Nov 3 11:47:18 2017 +0000
|
|     Append fuga
|
* commit 7706f3beea43965c3dd92a9fcc914bbe4f91efbf
  Author: Yuki INOUE <my.mail@domain.com>
  Date:   Fri Nov 3 11:47:18 2017 +0000

      Initial

# 問題なければ確定する
( cd repo2
  git filter-branch -- --all
  git log --graph > ../repo2_rewritten.txt
  git push repo1 master:repo2-master
)

( cd repo1
  git log --graph repo2-master > ../repo1_new_master.txt
)

* commit 0477c8800097110a858834f5711b2fcee1380cbc
| Author: Yuki INOUE <my.mail@domain.com>
| Date:   Fri Nov 3 11:47:18 2017 +0000
|
|     Append piyo
|
* commit 87ee5eb689e4aa33c4b6816c501d89ee154658ea
| Author: Yuki INOUE <my.mail@domain.com>
| Date:   Fri Nov 3 11:47:18 2017 +0000
|
|     Append fuga
|
* commit 7706f3beea43965c3dd92a9fcc914bbe4f91efbf
  Author: Yuki INOUE <my.mail@domain.com>
  Date:   Fri Nov 3 11:47:18 2017 +0000

      Initial