pack-objects: Print a message describing the number of threads for packing

Signed-off-by: Brandon Casey <casey@nrlssc.navy.mil>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin-pack-objects.c

@@ -2239,6 +2239,9 @@ int cmd_pack_objects(int argc, const char **argv, const char *prefix)
 #ifdef THREADED_DELTA_SEARCH
 	if (!delta_search_threads)	/* --threads=0 means autodetect */
 		delta_search_threads = online_cpus();
+	if (progress)
+		fprintf(stderr, "Using %d pack threads.\n",
+			delta_search_threads);
 #endif
 
 	prepare_packed_git();