After doing a rebase, apply git filter-branch to keep original committer/author dates

git-rebase-keep-dates.sh

# Very important assumption: you have commit messages with _unique_ subject lines ('%s')
# If not, you need to either modify my gist or "go back to the drawing" board…
# Use these to check if you have unique subject lines or not:
# (the numbers must be identical)

export BRANCH=master
export OLD_BRANCH=old_branch

git log --format="%s" "$BRANCH" | uniq | wc -l | xargs
git log --oneline "$BRANCH" | wc -l | xargs

# Before doing these changes, create a backup!

git filter-branch --force --env-filter 'GIT_COMMITTER_DATE=$(git log --format='%cd' --grep="$(git show --format='%s' --no-patch $GIT_COMMIT)" "$OLD_BRANCH")' "$BRANCH"

# Inspired by:
# https://codewithhugo.com/change-the-date-of-a-git-commit/
# https://stackoverflow.com/questions/30790645/how-to-make-a-git-rebase-and-keep-the-commit-timestamp