Show a git branch

[certik]

This one is tricky: we do not want to touch the file system after every "enter". Rather, it looks like we need to be watching .git/HEAD in a separate thread, and update an internal variable every time the git branch changes. Then in the prompt we want to just print the branch.

Later we should change the git branch right when it happens (no need to press enter).

[certik]

Fixed in #67