Teach bash to complete ref arguments to git-describe

Teach bash to complete ref arguments to git-describe

I'm often finding that I need to run git-describe on very long
remote tracking branch names, to find out what tagged revision
the remote tracking branch is now at (or not at).  Typing out
the ref names is painful, so bash completion on them is a very
useful feature.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>