git-phantom

#!/bin/bash
#
# Create an empty commit reusing author, date and message from an existing commit.
#
# Usage:
#   git phantom <commit-ish>
#
set -euo pipefail

if [ $# -ne 1 ]; then
  echo "Usage: git phantom <commit-ish>" >&2
  exit 1
fi

if ! git diff --cached --quiet; then
  echo "Error: You have staged changes." >&2
  exit 1
fi

commit=$1

if ! git rev-parse --verify -q "$commit" >/dev/null; then
  echo "Error: Not a valid commit-ish: $commit" >&2
  exit 1
fi

author="$(git show -s --format='%an <%ae>' "$commit")"
author_date="$(git show -s --format='%aI' "$commit")"
commit_message="$(git show -s --format='%B' "$commit")"

args=(--allow-empty --author="$author")
if [ -z "$commit_message" ]; then
  args+=(--allow-empty-message -m "")
else
  args+=(-m "$commit_message")
fi

GIT_AUTHOR_DATE="$author_date" git commit "${args[@]}"