Clear the delta base cache if a pack is rebuilt

There is some risk that re-opening a regenerated pack file with
different offsets could leave stale entries within the delta base
cache that could be matched up against other objects using the same
"struct packed_git*" and pack offset.

Throwing away the entire delta base cache in this case is safer,
as we don't have to worry about a recycled "struct packed_git*"
matching to the wrong base object, resulting in delta apply
errors while unpacking an object.

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

sha1_file.c

@@ -676,6 +676,7 @@ void free_pack_by_name(const char *pack_name)
 	while (*pp) {
 		p = *pp;
 		if (strcmp(pack_name, p->pack_name) == 0) {
+			clear_delta_base_cache();
 			close_pack_windows(p);
 			if (p->pack_fd != -1)
 				close(p->pack_fd);