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>

object.h

@@ -26,6 +26,19 @@ struct object_array {
 #define OBJECT_ARRAY_INIT { 0, 0, NULL }
 
 #define TYPE_BITS   3
+/*
+ * object flag allocation:
+ * revision.h:      0---------10
+ * fetch-pack.c:    0---4
+ * walker.c:        0-2
+ * upload-pack.c:               11----------------19
+ * builtin/blame.c:               12-13
+ * bisect.c:                               16
+ * bundle.c:                               16
+ * http-push.c:                            16-----19
+ * commit.c:                               16-----19
+ * sha1_name.c:                                     20
+ */
 #define FLAG_BITS  27
 
 /*