parse-remote: function to get the tracking branch to be merge

parse-remote: function to get the tracking branch to be merge

The only user of get_remote_refs_for_fetch was "git pull --rebase" and
it only wanted the tracking branch to be merge. So, add a simple
function (get_remote_merge_branch) with this new meaning.

No behavior changes. The new function behaves like the old code in
"git pull --rebase". In particular, it only works with the default
refspec mapping and with remote branches, not tags.

Signed-off-by: Santi Béjar <santi@agolina.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>