tree-walk: Correct bitrotted comment about tree_entry()

There was a code comment that referred to the "above two functions" but
over time the functions immediately preceding the comment have changed.
Just mention the relevant functions by name.
Signed-off-by: NElijah Newren <newren@gmail.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

tree-walk.h

@@ -28,7 +28,10 @@ static inline int tree_entry_len(const char *name, const unsigned char *sha1)
 void update_tree_entry(struct tree_desc *);
 void init_tree_desc(struct tree_desc *desc, const void *buf, unsigned long size);
 
-/* Helper function that does both of the above and returns true for success */
+/*
+ * Helper function that does both tree_entry_extract() and update_tree_entry()
+ * and returns true for success
+ */
 int tree_entry(struct tree_desc *, struct name_entry *);
 
 void *fill_tree_descriptor(struct tree_desc *desc, const unsigned char *sha1);