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: NDaniel Barkalow <barkalow@iabervon.org>
Signed-off-by: NShawn O. Pearce <spearce@spearce.org>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

cache.h

@@ -739,6 +739,7 @@ extern unsigned char* use_pack(struct packed_git *, struct pack_window **, off_t
 extern void close_pack_windows(struct packed_git *);
 extern void unuse_pack(struct pack_window **);
 extern void free_pack_by_name(const char *);
+extern void clear_delta_base_cache(void);
 extern struct packed_git *add_packed_git(const char *, int, int);
 extern const unsigned char *nth_packed_object_sha1(struct packed_git *, uint32_t);
 extern off_t nth_packed_object_offset(const struct packed_git *, uint32_t);

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;

sha1_file.c

@@ -1630,6 +1630,13 @@ static inline void release_delta_base_cache(struct delta_base_cache_entry *ent)
 	}
 }
 
+void clear_delta_base_cache(void)
+{
+	unsigned long p;
+	for (p = 0; p < MAX_DELTA_CACHE; p++)
+		release_delta_base_cache(&delta_base_cache[p]);
+}
+
 static void add_delta_base_cache(struct packed_git *p, off_t base_offset,
 	void *base, unsigned long base_size, enum object_type type)
 {