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>

builtin-count-objects.c

@@ -62,7 +62,7 @@ static void count_objects(DIR *d, char *path, int len, int verbose,
 		hex[40] = 0;
 		if (get_sha1_hex(hex, sha1))
 			die("internal error");
-		if (has_sha1_pack(sha1))
+		if (has_sha1_pack(sha1, NULL))
 			(*packed_loose)++;
 	}
 }