revision traversal: retire BOUNDARY_SHOW

This removes the flag internally used by revision traversal to
decide which commits are indeed boundaries and renames it to
CHILD_SHOWN.  builtin-bundle uses the symbol for its
verification, but I think the logic it uses it is wrong.  The
flag is still useful but it is local to the git-bundle, so it is
renamed to PREREQ_MARK.

Signed-off-by: Junio C Hamano <junkio@cox.net>
diff --git a/revision.h b/revision.h
index 6579a44..1885f8d 100644
--- a/revision.h
+++ b/revision.h
@@ -7,10 +7,9 @@
 #define SHOWN		(1u<<3)
 #define TMP_MARK	(1u<<4) /* for isolated cases; clean after use */
 #define BOUNDARY	(1u<<5)
-#define BOUNDARY_SHOW	(1u<<6)
+#define CHILD_SHOWN	(1u<<6)
 #define ADDED		(1u<<7)	/* Parents already parsed and added? */
 #define SYMMETRIC_LEFT	(1u<<8)
-#define CHILD_SHOWN	(1u<<9)
 
 struct rev_info;
 struct log_info;