get_sha1_basic(): try refs/... and finally refs/remotes/$foo/HEAD

get_sha1_basic(): try refs/... and finally refs/remotes/$foo/HEAD

This implements the suggestion by Jeff King to use
refs/remotes/$foo/HEAD to interpret a shorthand "$foo" to mean
the primary branch head of a tracked remote.  clone needs to be
told about this convention as well.

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