Expand get_remote_merge_branch to compute the tracking branch to merge
when called without arguments (or only the remote name). This allows
"git pull --rebase" without arguments (default upstream branch) to
work with a rebased upstream. With explicit arguments it already worked.
Also add a test to check for this case.
Signed-off-by: Santi Béjar <santi@agolina.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>