http-push: support for updating remote info/refs

If info/refs exists on the remote, get a lock on info/refs, make sure that
there is a local copy of the object referenced in each remote ref (in case
someone else added a tag we don't have locally), do all the refspec updates,
and generate and send an updated info/refs file.

Signed-off-by: Junio C Hamano <junkio@cox.net>