fetch: fix following tags when fetching specific OID

fetch: fix following tags when fetching specific OID

In 3f763ddf28 (fetch: set remote/HEAD if it does not exist, 2024-11-22),
unconditionally adds "HEAD" to the list of ref prefixes we send to the
server.

This breaks a core assumption that the list of prefixes we send to the
server is complete. We must either send all prefixes we care about, or
none at all (in the latter case the server then advertises everything).

The tag following code is careful to only add "refs/tags/" to the list
of prefixes if there are already entries in the prefix list. But because
the new code from 3f763ddf28 runs after the tag code, and because it
unconditionally adds to the prefix list, we may end up with a prefix
list that _should_ have "refs/tags/" in it, but doesn't.

When that is the case, the server does not advertise any tags, and our
auto-following breaks because we never learned about any tags in the
first place.

Fix this by only adding "HEAD" to the ref prefixes when we know that we
are already limiting the advertisement. In either case we'll learn about
HEAD (either through the limited advertisement, or implicitly through a
full advertisement).

Reported-by: Igor Todorovski <itodorov@ca.ibm.com>
Co-authored-by: Jeff King <peff@peff.net>
Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>