tree/diff header cleanup.

Introduce tree-walk.[ch] and move "struct tree_desc" and
associated functions from various places.

Rename DIFF_FILE_CANON_MODE(mode) macro to canon_mode(mode) and
move it to cache.h.  This macro returns the canonicalized
st_mode value in the host byte order for files, symlinks and
directories -- to be compared with a tree_desc entry.
create_ce_mode(mode) in cache.h is similar but is intended to be
used for index entries (so it does not work for directories) and
returns the value in the network byte order.

Signed-off-by: Junio C Hamano <junkio@cox.net>

diff.h

@@ -4,17 +4,7 @@
 #ifndef DIFF_H
 #define DIFF_H
 
-#define DIFF_FILE_CANON_MODE(mode) \
-	(S_ISREG(mode) ? (S_IFREG | ce_permissions(mode)) : \
-	S_ISLNK(mode) ? S_IFLNK : S_IFDIR)
-
-struct tree_desc {
-	void *buf;
-	unsigned long size;
-};
-
-extern void update_tree_entry(struct tree_desc *);
-extern const unsigned char *tree_entry_extract(struct tree_desc *, const char **, unsigned int *);
+#include "tree-walk.h"
 
 struct diff_options;