packed_ref_iterator_begin(): make optimization more general

We can return an empty iterator not only if the `packed-refs` file is
missing, but also if it is empty or if there are no references whose
names succeed `prefix`. Optimize away those cases as well by moving
the call to `find_reference_location()` higher in the function and
checking whether the determined start position is the same as
`snapshot->eof`. (This is possible now because the previous commit
made `find_reference_location()` robust against empty snapshots.)

Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

refs/packed-backend.c

@@ -927,7 +927,12 @@ static struct ref_iterator *packed_ref_iterator_begin(
 	 */
 	snapshot = get_snapshot(refs);
 
-	if (!snapshot->buf)
+	if (prefix && *prefix)
+		start = find_reference_location(snapshot, prefix, 0);
+	else
+		start = snapshot->start;
+
+	if (start == snapshot->eof)
 		return empty_ref_iterator_begin();
 
 	iter = xcalloc(1, sizeof(*iter));
@@ -937,11 +942,6 @@ static struct ref_iterator *packed_ref_iterator_begin(
 	iter->snapshot = snapshot;
 	acquire_snapshot(snapshot);
 
-	if (prefix && *prefix)
-		start = find_reference_location(snapshot, prefix, 0);
-	else
-		start = snapshot->start;
-
 	iter->pos = start;
 	iter->eof = snapshot->eof;
 	strbuf_init(&iter->refname_buf, 0);