Only prompt git branch when in repo root.

Always showing the git branch in my bash prompt was bugging me; I have
my home directory under git, so every subdirectory under it had a git
branch in the prompt, even when _not_ under version control.

Obvious solutions that first came to mind:

    - Look for ignored directories and don't show the branch if `pwd` is
      in there.
    - Don't bother with branches if no git dir in the current directory.

I chose the latter because in my flow, I think I only really look at the
branch in the prompt when in a project's root directory... time will
tell... if this system doesn't cut it, I'll try the .gitignore approach.