How to automatically prepend git commit with a branch number tag

prepare-commit-msg.sh

#!/bin/bash
#Does not work if executed as sh
#Copy this commit script into prepare-commit-msg

BRANCH_NAME=$(git branch 2>/dev/null | grep -e ^* | tr -d ' *')
IFS='-' read -r -a array <<< "$BRANCH_NAME"
if [ -n "$BRANCH_NAME" ]; then
    echo " (#$array) $(cat $1)" > $1
fi