git bash prompt

There are a few styles flying around the internet and I just wanted to contribute mine. It is based on the one I've found here: http://mediadoneright.com/content/ultimate-git-ps1-bash-prompt - with some minor modifications:

• The prompt's color scheme is made for white letters on black background
• The path is shortened
• Beside you branch the short sha1 of your HEAD is displayed
• Replaced the time with user@host