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>

pack-objects.c

@@ -3,7 +3,7 @@
 #include "delta.h"
 #include "pack.h"
 #include "csum-file.h"
-#include "diff.h"
+#include "tree-walk.h"
 #include <sys/time.h>
 #include <signal.h>