How to automatically prepend git commit with a branch name

prepare-commit-msg.sh

#!/bin/bash

# $1 is the commit message

# This way you can customize which branches should be skipped when
# prepending commit message. 
if [ -z "$BRANCHES_TO_SKIP" ]; then
  BRANCHES_TO_SKIP=(master develop stage)
fi

BRANCH_NAME=$(git symbolic-ref --short HEAD)
# ${string##substring}: Deletes longest match of $substring from front of $string.
# http://tldp.org/LDP/abs/html/string-manipulation.html
BRANCH_NAME="${BRANCH_NAME##*/}"

BRANCH_EXCLUDED=$(printf "%s\n" "${BRANCHES_TO_SKIP[@]}" | grep -c "^$BRANCH_NAME$")
BRANCH_IN_COMMIT=$(grep -c "\[$BRANCH_NAME\]" $1)

# -n : string length > 0
if [ -n "$BRANCH_NAME" ] && ! [[ $BRANCH_EXCLUDED -eq 1 ]] && ! [[ $BRANCH_IN_COMMIT -ge 1 ]]; then 
  sed -i.bak -e "1s/^/[$BRANCH_NAME] /" $1
fi