How do I implement a .diff changeset from core.trac.wordpress.org locally?

The command you need is patch which is not specific to SVN. It’s really easy to do. See this article: http://jungels.net/articles/diff-patch-ten-minutes.html

Hata!: SQLSTATE[HY000] [1045] Access denied for user 'divattrend_liink'@'localhost' (using password: YES)