Jeff King [off-list ref] writes:

 	if (!cb.reccnt) {
+		if (cnt == 0) {

Style "if (!cnt)" ?  In this particular case I do not think it
actually is an improvement, though, simply because zero is really
special in this logic.

+			/*
+			 * The caller asked for ref@{0}, and we had no entries.
+			 * It's a bit subtle, but in practice all callers have
+			 * prepped the "oid" field with the current value of
+			 * the ref, which is the most reasonable fallback.
+			 *
+			 * We'll put dummy values into the out-parameters (so
+			 * they're not just uninitialized garbage), and the
+			 * caller can take our return value as a hint that
+			 * we did not find any such reflog.
+			 */
+			set_read_ref_cutoffs(&cb, 0, 0, "empty reflog");
+			return 1;
+		}

The dummy value I 100% agree with ;-).

You mentioned the convenience special case for time-based reflog
query for a time older than (e.g. @{20.years.ago}) the reflog
itself, and perhaps this one should be treated as its counterpart,
that is only useful for count-based access.