pack-write: skip *.rev work when not writing *.rev

Fix a performance regression introduced in a587b5a (pack-write.c:
extract 'write_rev_file_order', 2021-03-30) and stop needlessly
allocating the "pack_order" array and sorting it with
"pack_order_cmp()", only to throw that work away when we discover that
we're not writing *.rev files after all.

This redundant work was not present in the original version of this
code added in 8ef50d9 (pack-write.c: prepare to write 'pack-*.rev'
files, 2021-01-25). There we'd call write_rev_file() from
e.g. finish_tmp_packfile(), but we'd "return NULL" early in
write_rev_file() if not doing a "WRITE_REV" or "WRITE_REV_VERIFY".

Signed-off-by: Ævar Arnfjörð Bjarmason <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: avar

pack-write.c

@@ -224,6 +224,9 @@ const char *write_rev_file(const char *rev_name,
 	uint32_t i;
 	const char *ret;
 
+	if (!(flags & WRITE_REV) && !(flags & WRITE_REV_VERIFY))
+		return NULL;
+
 	ALLOC_ARRAY(pack_order, nr_objects);
 	for (i = 0; i < nr_objects; i++)
 		pack_order[i] = i;