tree-diff: rename compare_tree_entry -> tree_entry_pathcmp

tree-diff: rename compare_tree_entry -> tree_entry_pathcmp

Since previous commit, this function does not compare entry hashes, and
mode are compared fully outside of it. So what it does is compare entry
names and DIR bit in modes. Reflect this in its name.

Add documentation stating the semantics, and move the note about
files/dirs comparison to it.

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