Two trivial -Wcast-qual fixes

Luiz Fernando N. Capitulino noticed the one in tree-walk.h where
we cast away constness while computing the legnth of a tree
entry.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

cache-tree.c

@@ -478,7 +478,7 @@ static struct cache_tree *read_one(const char **buffer, unsigned long *size_p)
 	if (0 <= it->entry_count) {
 		if (size < 20)
 			goto free_return;
-		hashcpy(it->sha1, (unsigned char*)buf);
+		hashcpy(it->sha1, (const unsigned char*)buf);
 		buf += 20;
 		size -= 20;
 	}

tree-walk.h

@@ -22,7 +22,7 @@ static inline const unsigned char *tree_entry_extract(struct tree_desc *desc, co
 
 static inline int tree_entry_len(const char *name, const unsigned char *sha1)
 {
-	return (char *)sha1 - (char *)name - 1;
+	return (const char *)sha1 - name - 1;
 }
 
 void update_tree_entry(struct tree_desc *);