remote.c: hoist branch.*.remote lookup out of remote_get_1

remote.c: hoist branch.*.remote lookup out of remote_get_1

We'll want to use this logic as a fallback when looking up
the pushremote, so let's pull it out into its own function.

We don't technically need to make this available outside of
remote.c, but doing so will provide a consistent API with
pushremote_for_branch, which we will add later.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>