Resolve value supplied for no-colon push refspecs

Resolve value supplied for no-colon push refspecs

When pushing a refspec like "HEAD", we used to treat it as
"HEAD:HEAD", which didn't work without rewriting. Instead, we should
resolve the ref. If it's a symref, further require it to point to a
branch, to avoid doing anything especially unexpected. Also remove the
rewriting previously added in builtin-push.

Since the code for "HEAD" uses the regular refspec parsing, it
automatically handles "+HEAD" without anything special.

[jc: added a further test to make sure that "remote.*.push = HEAD" works]

Signed-off-by: Daniel Barkalow <barkalow@iabervon.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>