Clear the delta base cache during fast-import checkpoint

Otherwise we may reuse the same memory address for a totally
different "struct packed_git", and a previously cached object from
the prior occupant might be returned when trying to unpack an object
from the new pack.

Found-by: Daniel Barkalow <barkalow@iabervon.org>
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

fast-import.c

@@ -943,6 +943,7 @@ static void end_packfile(void)
 {
 	struct packed_git *old_p = pack_data, *new_p;
 
+	clear_delta_base_cache();
 	if (object_count) {
 		char *idx_name;
 		int i;