push: support pushing HEAD to real branch name
This teaches "push <remote> HEAD" to resolve HEAD on the local
side to its real branch name, e.g. master, and then act as if
the real branch name was specified. So we have a shorthand for
pushing the current branch. Besides HEAD, no other symbolic ref
is resolved.
Thanks to Daniel Barkalow <barkalow@iabervon.org> for suggesting
this implementation, which is much simpler than the
implementation proposed before.
Signed-off-by: Steffen Prohaska <prohaska@zib.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>