builtin/remote: determine whether refs need renaming early on

builtin/remote: determine whether refs need renaming early on

When renaming a remote we may have to also rename remote refs in case
the refspec changes. Pull out this computation into a separate loop.
While that seems nonsensical right now, it'll help us in a subsequent
commit where we will prepare the reference transaction before we rewrite
the configuration.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>