tree-walk: finally switch over tree descriptors to contain a pre-parsed entry

This continues 4651ece8 (Switch over tree descriptors to contain a
pre-parsed entry) and moves the only rest computational part

    mode = canon_mode(mode)

from tree_entry_extract() to tree entry decode phase - to
decode_tree_entry().

The reason to do it, is that canon_mode() is at least 2 conditional
jumps for regular files, and that could be noticeable should canon_mode()
be invoked several times.

That does not matter for current Git codebase, where typical tree
traversal is

    while (t->size) {
        sha1 = tree_entry_extract(t, &path, &mode);
        ...
        update_tree_entry(t);
    }

i.e. we do t -> sha1,path.mode "extraction" only once per entry. In such
cases, it does not matter performance-wise, where that mode
canonicalization is done - either once in tree_entry_extract(), or once
in decode_tree_entry() called by update_tree_entry() - it is
approximately the same.

But for future code, which could need to work with several tree_desc's
in parallel, it could be handy to operate on tree_desc descriptors, and
do "extracts" only when needed, or at all, access only relevant part of
it through structure fields directly.

And for such situations, having canon_mode() be done once in decode
phase is better - we won't need to pay the performance price of 2 extra
conditional jumps on every t->mode access.

So let's move mode canonicalization to decode_tree_entry(). That was the
final bit. Now after tree entry is decoded, it is fully ready and could
be accessed either directly via field, or through tree_entry_extract()
which this time got really "totally trivial".

Signed-off-by: Kirill Smelkov <kirr@mns.spb.ru>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

tree-walk.c

@@ -37,7 +37,7 @@ static void decode_tree_entry(struct tree_desc *desc, const char *buf, unsigned
 
 	/* Initialize the descriptor entry */
 	desc->entry.path = path;
-	desc->entry.mode = mode;
+	desc->entry.mode = canon_mode(mode);
 	desc->entry.sha1 = (const unsigned char *)(path + len);
 }
 

tree-walk.h

@@ -16,7 +16,7 @@ struct tree_desc {
 static inline const unsigned char *tree_entry_extract(struct tree_desc *desc, const char **pathp, unsigned int *modep)
 {
 	*pathp = desc->entry.path;
-	*modep = canon_mode(desc->entry.mode);
+	*modep = desc->entry.mode;
 	return desc->entry.sha1;
 }