pack-objects: write objects packed to trace2

This is useful when investigating performance of pushes, and other times
when no progress information is written (because the pack is written to
stdout).

Signed-off-by: Jonathan Tan <jonathantanmy@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/pack-objects.c

@@ -964,6 +964,8 @@ static void write_pack_file(void)
 	if (written != nr_result)
 		die(_("wrote %"PRIu32" objects while expecting %"PRIu32),
 		    written, nr_result);
+	trace2_data_intmax("pack-objects", the_repository,
+			   "write_pack_file/wrote", nr_result);
 }
 
 static int no_try_delta(const char *path)