Do not try futile object pairs when repacking.

In the repacking window, if both objects we are looking at already came
from the same (old) pack-file, don't bother delta'ing them against each
other.

That means that we'll still always check for better deltas for (and
against!) _unpacked_ objects, but assuming incremental repacks, you'll
avoid the delta creation 99% of the time.

Signed-off-by: Linus Torvalds <torvalds@osdl.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>

pack-objects.c

@@ -987,6 +987,13 @@ static int try_delta(struct unpacked *trg, struct unpacked *src,
 	if (trg_entry->preferred_base)
 		return -1;
 
+	/*
+	 * We do not bother to try a delta that we discarded
+	 * on an earlier try.
+	 */
+	if (trg_entry->in_pack && trg_entry->in_pack == src_entry->in_pack)
+		return 0;
+
 	/*
 	 * If the current object is at pack edge, take the depth the
 	 * objects that depend on the current object into account --