make pack data reuse compatible with both delta types

This is the missing part to git-pack-objects allowing it to reuse delta
data to/from any of the two delta types.  It can reuse delta from any
type, and it outputs base offsets when --allow-delta-base-offset is
provided and the base is also included in the pack.  Otherwise it
outputs base sha1 references just like it always did.

Signed-off-by: Nicolas Pitre <nico@cam.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>

pack.h

@@ -16,7 +16,4 @@ struct pack_header {
 };
 
 extern int verify_pack(struct packed_git *, int);
-extern int check_reuse_pack_delta(struct packed_git *, unsigned long,
-				  unsigned char *, unsigned long *,
-				  enum object_type *);
 #endif