promisor-remote: refactor should_accept_remote() control flow

promisor-remote: refactor should_accept_remote() control flow

A previous commit made sure we now reject empty URLs early at parse
time. This makes the existing warning() in case a remote URL is NULL
or empty very unlikely to be useful.

In future work, we also plan to add URL-based acceptance logic into
should_accept_remote().

To adapt to previous changes and prepare for upcoming changes, let's
restructure the control flow in should_accept_remote().

Concretely, let's:

 - Replace the warning() in case of an empty URL with a BUG(), as a
   previous commit made sure empty URLs are rejected early at parse
   time.

 - Move that modified empty-URL check to the very top of the function,
   so that every acceptance mode, instead of only ACCEPT_KNOWN_URL, is
   covered.

 - Invert the URL comparison: instead of returning on match and
   warning on mismatch, return early on mismatch and let the match
   case fall through. This opens a single exit path at the bottom of
   the function for future commits to extend.

Signed-off-by: Christian Couder <chriscool@tuxfamily.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>