get_oid_basic(): special-case ref@{n} for oldest reflog entry

The goal of 6436a20284 (refs: allow @{n} to work with n-sized reflog,
2021-01-07) was that if we have "n" entries in a reflog, we should still
be able to resolve ref@{n} by looking at the "old" value of the oldest
entry.

Commit 6436a20284 tried to put the logic into read_ref_at() by shifting
its idea of "n" by one. But we reverted that in the previous commit,
since it led to bugs in other callers which cared about the details of
the reflog entry we found. Instead, let's put the special case into the
caller that resolves @{n}, as it cares only about the oid.

read_ref_at() is even kind enough to return the "old" value from the
final reflog; it just returns "1" to signal to us that we ran off the
end of the reflog. But we can notice in the caller that we read just
enough records for that "old" value to be the one we're looking for, and
use it.

Note that read_ref_at() could notice this case, too, and just return 0.
But we don't want to do that, because the caller must be made aware that
we only found the oid, not an actual reflog entry (and the call sites in
show-branch do care about this).

There is one complication, though. When read_ref_at() hits a truncated
reflog, it will return the "old" value of the oldest entry only if it is
not the null oid. Otherwise, it actually returns the "new" value from
that entry! This bit of fudging is due to d1a4489a56 (avoid null SHA1 in
oldest reflog, 2008-07-08), where asking for "ref@{20.years.ago}" for a
ref created recently will produce the initial value as a convenience
(even though technically it did not exist 20 years ago).

But this convenience is only useful for time-based cutoffs. For
count-based cutoffs, get_oid_basic() has always simply complained about
going too far back:

  $ git rev-parse HEAD@{20}
  fatal: log for 'HEAD' only has 16 entries

and we should continue to do so, rather than returning a nonsense value
(there's even a test in t1508 already which covers this). So let's have
the d1a4489a56 code kick in only when doing timestamp-based cutoffs.

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

refs.c

@@ -1083,7 +1083,7 @@ static int read_ref_at_ent_oldest(struct object_id *ooid, struct object_id *noid
 
 	set_read_ref_cutoffs(cb, timestamp, tz, message);
 	oidcpy(cb->oid, ooid);
-	if (is_null_oid(cb->oid))
+	if (cb->at_time && is_null_oid(cb->oid))
 		oidcpy(cb->oid, noid);
 	/* We just want the first entry */
 	return 1;