git-changed-in-head helper script

A simple bash wrapper to easily list modified files in HEAD that matches
a set of extensions. It will NOT list deleted files.

Examples:

 git-changed-in-head php php5 inc
 git-changed-in-head pp
 git-changed-in-head js

Change-Id: If91cecca4e94db88939c8a1978430085ab3279e0