Keep a list of the hashes by which a commit was previously known

update_commit_previous_hashes_list

#!/usr/bin/env sh

# A script to keep a list of the hashes by which a commit was previously known
# Adapted from https://stackoverflow.com/a/54352290/3324977

# Installation:
# - copy this script to a directory in your $PATH
# - make it executable:
#   chmod +x ./update_commit_previous_hashes_list
# - it should now be accessible from any directory, which can be checked like this:
#   cd some/other/directory
#   which update_commit_previous_hashes_list

# Usage:
# - rebase on a branch named 'some-branch'
#   git rebase some-branch --exec update_commit_previous_hashes_list
# - same but in interactive mode: you should see an exec command between every pick
#   git rebase some-branch --interactive --exec update_commit_previous_hashes_list

# Get the message of the commit we just rebased
git log --pretty="format:%B" -n 1 > _last_rebased_commit_message

metadata_header="This commit was previously known as:"

# If the header is already there, there is already a list of previous commit hashes,
# no need to re-add that header
grep "$metadata_header" _last_rebased_commit_message > /dev/null || {
  echo "\n$metadata_header" >> _last_rebased_commit_message
}

# Get the hash of the commit we just rebased
grep --only-matching "\w\{40\}" .git/rebase-merge/done | tail -n 1 >> _last_rebased_commit_message

# Update the commit message
git commit --amend --file _last_rebased_commit_message

# Cleanup
rm _last_rebased_commit_message

This is especially useful when some commits refers to some other commits by hash, and that hash would have otherwise been lost during the rebase: that commit can now by found using git log --grep

git log --grep 9046a6d6