parse-remote: handle detached HEAD

get_remote_merge_branch with zero or one arguments returns the
upstream branch. But a detached HEAD does no have an upstream branch,
as it is not tracking anything. Handle this case testing the exit code
of "git symbolic-ref -q HEAD".

Reported-by: Sverre Rabbelier <srabbelier@gmail.com>
Signed-off-by: Santi Béjar <santi@agolina.net>
Tested-by: Sverre Rabbelier <srabbelier@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>