simple euristic for further free packing improvements

Given that the early eviction of objects with maximum delta depth
may exhibit bad packing on its own, why not considering a bias against
deep base objects in try_delta() to mitigate that bad behavior.

This patch adjust the MAX_size allowed for a delta based on the depth of
the base object as well as enabling the early eviction of max depth
objects from the object window.  When used separately, those two things
produce slightly better and much worse results respectively.  But their
combined effect is a surprising significant packing improvement.

With this really simple patch the GIT repo gets nearly 15% smaller, and
the Linux kernel repo about 5% smaller, with no significantly measurable
CPU usage difference.

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

pack-objects.c

@@ -1039,8 +1039,8 @@ static int try_delta(struct unpacked *trg, struct unpacked *src,
 
 	/* Now some size filtering euristics. */
 	size = trg_entry->size;
-	max_size = size / 2 - 20;
-	if (trg_entry->delta)
+	max_size = (size/2 - 20) / (src_entry->depth + 1);
+	if (trg_entry->delta && trg_entry->delta_size <= max_size)
 		max_size = trg_entry->delta_size-1;
 	src_size = src_entry->size;
 	sizediff = src_size < size ? size - src_size : 0;
@@ -1129,15 +1129,12 @@ static void find_deltas(struct object_entry **list, int window, int depth)
 			if (try_delta(n, m, m->index, depth) < 0)
 				break;
 		}
-#if 0
 		/* if we made n a delta, and if n is already at max
 		 * depth, leaving it in the window is pointless.  we
 		 * should evict it first.
-		 * ... in theory only; somehow this makes things worse.
 		 */
 		if (entry->delta && depth <= entry->depth)
 			continue;
-#endif
 		idx++;
 		if (idx >= window)
 			idx = 0;