improve reliability of fixup_pack_header_footer()

Currently, this function has the potential to read corrupted pack data
from disk and give it a valid SHA1 checksum.  Let's add the ability to
validate SHA1 checksum of existing data along the way, including before
and after any arbitrary point in the pack.

Signed-off-by: Nicolas Pitre <nico@cam.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

index-pack.c

@@ -982,7 +982,8 @@ int main(int argc, char **argv)
 				nr_objects - nr_objects_initial);
 			stop_progress_msg(&progress, msg);
 			fixup_pack_header_footer(output_fd, sha1,
-						 curr_pack, nr_objects);
+						 curr_pack, nr_objects,
+						 NULL, 0);
 		}
 		if (nr_deltas != nr_resolved_deltas)
 			die("pack has %d unresolved deltas",