git-completion.bash up to version shipped w/ git-1.7.2.1

I really need to find a better way of managing this. Right now we
have a bunch of source level hacks for dealing with commands aliased
to top-level. It looks like git-completion.bash has abstracted
detection of expansions behind a few functions. We could possibly
overwrite those functions with custom versions that makes out
gitalias commands available.