Add a "max_size" parameter to diff_delta()

Anything that generates a delta to see if two objects are close usually
isn't interested in the delta ends up being bigger than some specified
size, and this allows us to stop delta generation early when that
happens.

diff-delta.c

@@ -203,7 +203,8 @@ static void delta_cleanup(bdfile_t *bdf)
 
 void *diff_delta(void *from_buf, unsigned long from_size,
 		 void *to_buf, unsigned long to_size,
-		 unsigned long *delta_size)
+		 unsigned long *delta_size,
+		 unsigned long max_size)
 {
 	int i, outpos, outsize, inscnt, csize, msize, moff;
 	unsigned int fp;
@@ -312,6 +313,11 @@ void *diff_delta(void *from_buf, unsigned long from_size,
 		}
 
 		/* next time around the largest possible output is 1 + 4 + 3 */
+		if (max_size && outpos > max_size) {
+			free(out);
+			delta_cleanup(&bdf);
+			return NULL;
+		}
 		if (outpos > outsize - 8) {
 			void *tmp = out;
 			outsize = outsize * 3 / 2;