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>

cache.h

@@ -106,6 +106,9 @@ static inline unsigned int create_ce_mode(unsigned int mode)
 		return htonl(S_IFLNK);
 	return htonl(S_IFREG | ce_permissions(mode));
 }
+#define canon_mode(mode) \
+	(S_ISREG(mode) ? (S_IFREG | ce_permissions(mode)) : \
+	S_ISLNK(mode) ? S_IFLNK : S_IFDIR)
 
 #define cache_entry_size(len) ((offsetof(struct cache_entry,name) + (len) + 8) & ~7)