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

#!/bin/bash

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" )

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

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

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

printf '%s %sn' $repo2_init $repo1_end >> repo2/.git/info/grafts

git_log_pretty() {
    git log --graph "$@"
}

( cd repo2
  git_log_pretty )

* commit 70539c2d1d284d7487d6de8e1fef4919a25308f2
| Author: Yuki INOUE <my.mail@domain.com>
| Date:   Fri Nov 3 11:22:55 2017 +0000
|
|     Append piyo
|
* commit b0a1ac7d9935e254c2344d45fb3dfaaba9cd108d
| Author: Yuki INOUE <my.mail@domain.com>
| Date:   Fri Nov 3 11:22:55 2017 +0000
|
|     Re-initialize
|
* commit ee8b813b9bb5d6fbef6b813fa3ac2cca73b5f993
| Author: Yuki INOUE <my.mail@domain.com>
| Date:   Fri Nov 3 11:22:55 2017 +0000
|
|     Append fuga
|
* commit 466a183f763c35db676b89e644d055cab1a6dec5
  Author: Yuki INOUE <my.mail@domain.com>
  Date:   Fri Nov 3 11:22:55 2017 +0000

      Initial