Add `git diff --cached` output as a comment to commit messages

prepare-commit-msg

#!/bin/bash

commit_message_file="$1"
commit_type="$2"
commit_hash="$3" # This is empty if an existing commit is not being amended

diff_cached_output=$(git diff --cached)
diff_cached_output=$(echo "$diff_cached_output" | sed 's/^/#   /')

diff_cached_message="# \`git diff --cached\` output:

$diff_cached_output"


# Quote a string to be used as substitution for a `sed` program
quoteSedSubstitution() {
  IFS= read -d '' -r < <(sed -e ':a' -e '$!{N;ba' -e '}' -e 's/[&/\]/\\&/g; s/\n/\\&/g' <<<"$1")
  printf %s "${REPLY%$'\n'}"
}

sed -i -e "\$a$(quoteSedSubstitution "$diff_cached_message")" "$commit_message_file"