#!/usr/bin/env bash
# rewrite default merge subject to `merge: <branch with dashes as spaces>`

msg_file="$1"
source="$2"

[[ "$source" == "merge" ]] || exit 0

subject=$(head -n1 "$msg_file")

if [[ "$subject" =~ ^Merge\ remote-tracking\ branch\ \'[^/]+/([^\']+)\' ]]; then
	branch="${BASH_REMATCH[1]}"
elif [[ "$subject" =~ ^Merge\ branch\ \'([^\']+)\' ]]; then
	branch="${BASH_REMATCH[1]}"
else
	exit 0
fi

{
	printf 'merge: %s\n' "${branch//-/ }"
	tail -n +2 "$msg_file"
} > "$msg_file.tmp" && mv "$msg_file.tmp" "$msg_file"