object.h: centralize object flag allocation

While the field "flags" is mainly used by the revision walker, it is
also used in many other places. Centralize the whole flag allocation to
one place for a better overview (and easier to move flags if we have
too).

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/revision.h b/revision.h
index 88967d6..531fb0d 100644
--- a/revision.h
+++ b/revision.h
@@ -7,6 +7,7 @@
 #include "commit.h"
 #include "diff.h"
 
+/* Remember to update object flag allocation in object.h */
 #define SEEN		(1u<<0)
 #define UNINTERESTING   (1u<<1)
 #define TREESAME	(1u<<2)