$ git log --oneline --graph
*   7e26890 (HEAD -> master) Merge branch 'testbranch'
|
| * 56ef109 (testbranch) change 5 made
| * 1a78db3 change 4 made
| * d98b4cf change 3 made
| * e5e49af change 2 made
| * 8704c24 change 1 made
|/
* f318d97 added a.txt
* 252bf7f Initial commit

$ git log --oneline --graph
*   XXXYYGG (HEAD -> master) Merge branch 'testbranch'
|
| * XXXYYFF (testbranch) change 5 made
| * XXXYYEE change 4 made
| * XXXYYDD change 3 made
| * XXXYYCC change 2 made
| * XXXYYBB change 1 made
|/
* XXXYYAA added a.txt
* 8e99c2d Initial commit by Bob

#!/usr/bin/env bash

rm -rf oldrepo_git newrepo_git
mkdir oldrepo_git newrepo_git

cd oldrepo_git
git init
git config user.name tester
git config user.email tester@example.com
echo "# README" >> README
git add README
GIT_COMMITTER_DATE="1558960260" git commit --date "1558960260" -m "Initial commit"
echo "Testing" >> a.txt
git add a.txt
GIT_COMMITTER_DATE="1558960270" git commit --date "1558960270" -m "added a.txt"
git checkout -b testbranch
mkdir aa bb
for ix in 1 2 3 4 5; do
  echo $ix >> a.txt
  echo $ix >> b.txt
  echo $ix >> aa/aa.txt
  echo $ix >> aa/ab.txt
  git add .
  newts="$((1558960270+ix*10))"
  GIT_COMMITTER_DATE="$newts" git commit --date "$newts" -m "change $ix made"
done
git checkout master
ix="$((ix+1))"; newts="$((1558960270+ix*10))"
GIT_COMMITTER_DATE="$newts" GIT_AUTHOR_DATE="$newts" git merge --no-ff --no-edit testbranch

cd ../newrepo_git
git init
git config user.name bob
git config user.email bob@example.com
echo "# Bob's README" >> README
git add README
GIT_COMMITTER_DATE="1558960260" git commit --date "1558960260" -m "Initial commit by Bob"