packfile: close multi-pack-index in close_all_packs

Whenever we delete pack-files from the object directory, we need
to also delete the multi-pack-index that may refer to those
objects. Sometimes, this connection is obvious, like during a
repack. Other times, this is less obvious, like when gc calls
a repack command and then does other actions on the objects, like
write a commit-graph file.

The pattern we use to avoid out-of-date in-memory packed_git
structs is to call close_all_packs(). This should also call
close_midx(). Since we already pass an object store to
close_all_packs(), this is a nicely scoped operation.

This fixes a test failure when running t6500-gc.sh with
GIT_TEST_MULTI_PACK_INDEX=1.

Reported-by: Szeder Gábor <szeder.dev@gmail.com>
Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

packfile.c

@@ -339,6 +339,11 @@ void close_all_packs(struct raw_object_store *o)
 			BUG("want to close pack marked 'do-not-close'");
 		else
 			close_pack(p);
+
+	if (o->multi_pack_index) {
+		close_midx(o->multi_pack_index);
+		o->multi_pack_index = NULL;
+	}
 }
 
 /*