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: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

tree-walk.h

diff --git a/tree-walk.h b/tree-walk.h
index 42110a4..f78361a 100644
--- a/tree-walk.h
+++ b/tree-walk.h
@@ -28,7 +28,10 @@
 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);