show-branch: note about its object flags usage

This is another candidate for commit-slab. Keep Junio's observation in
code so we can search it later on when somebody wants to improve the
code.

Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/object.h b/object.h
index b8e70e5..caf3652 100644
--- a/object.h
+++ b/object.h
@@ -43,6 +43,7 @@ struct object_array {
  * builtin/index-pack.c:                                     2021
  * builtin/pack-objects.c:                                   20
  * builtin/reflog.c:                   10--12
+ * builtin/show-branch.c:    0-------------------------------------------26
  * builtin/unpack-objects.c:                                 2021
  */
 #define FLAG_BITS  27