pack-objects --unpacked=<existing pack> option.

Incremental repack without -a essentially boils down to:

	rev-list --objects --unpacked --all |
        pack-objects $new_pack

which picks up all loose objects that are still live and creates
a new pack.

This implements --unpacked=<existing pack> option to tell the
revision walking machinery to pretend as if objects in such a
pack are unpacked for the purpose of object listing.  With this,
we could say:

	rev-list --objects --unpacked=$active_pack --all |
	pack-objects $new_pack

instead, to mean "all live loose objects but pretend as if
objects that are in this pack are also unpacked".  The newly
created pack would be perfect for updating $active_pack by
replacing it.

Since pack-objects now knows how to do the rev-list's work
itself internally, you can also write the above example by:

	pack-objects --unpacked=$active_pack --all $new_pack </dev/null

Signed-off-by: Junio C Hamano <junkio@cox.net>
diff --git a/builtin-prune-packed.c b/builtin-prune-packed.c
index d3dd94d..960db49 100644
--- a/builtin-prune-packed.c
+++ b/builtin-prune-packed.c
@@ -19,7 +19,7 @@
 		memcpy(hex+2, de->d_name, 38);
 		if (get_sha1_hex(hex, sha1))
 			continue;
-		if (!has_sha1_pack(sha1))
+		if (!has_sha1_pack(sha1, NULL))
 			continue;
 		memcpy(pathname + len, de->d_name, 38);
 		if (dryrun)