My pre-commit hook

pre-commit

#!/bin/bash

debug=`git diff --cached | grep -C 2 -E '+\s*And (open|show me the page|save_and_open_page)'`
pry=`git diff --cached | grep -C 2 -E 'binding.pry'`
debugger=`git diff --cached | grep -C 2 debugger`
console_log=`git diff --cached | grep -C 2 console.log`
conflicts=`git diff --cached | grep -C 2 -E '<<<|>>>'`

if [ -n "$debug" ]; then
  echo "debugging statements added in this commit"
  echo $debug
fi

if [ -n "$pry" ]; then
  echo "binding.pry added in this commit"
  echo $debug
fi

if [ -n "$conflicts" ]; then
  echo "unresolved merge conflicts added in this commit"
  echo $conflicts
fi

if [ -n "$debugger" ]; then
  echo "JS debugging statements added in this commit"
  echo $debugger
fi

if [ -n "$console_log" ]; then
  echo "JS debugging statements added in this commit"
  echo $console_log
fi

if [[ -n "$conflicts" || -n "$debug" || -n "$debugger" || -n "$console_log" ]]; then
  exit 1
fi

http://book.git-scm.com/5_git_hooks.html

GIT_DIR in that link is a git repo, each repo will have its own hooks.