Don't try to delta if target is much smaller than source

Add a new try_delta heuristic.  Don't bother trying to make a delta if
the target object size is much smaller (currently 1/32) than the source,
as it's very likely not going to get a match.  Even if it does, you will
have to read at least 32x the size of the new file to reassemble it,
which isn't such a good deal.  This leads to a considerable performance
improvement when deltifying a mix of small and large files with a very
large window, because you don't have to wait for the large files to
percolate out of the window before things start going fast again.

Signed-off-by: Brian Downing <bdowning@lavos.net>
Acked-by: Nicolas Pitre <nico@cam.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin-pack-objects.c

@@ -1348,6 +1348,8 @@ static int try_delta(struct unpacked *trg, struct unpacked *src,
 	sizediff = src_size < trg_size ? trg_size - src_size : 0;
 	if (sizediff >= max_size)
 		return 0;
+	if (trg_size < src_size / 32)
+		return 0;
 
 	/* Load data if not already done */
 	if (!trg->data) {