Use non-greedy match when searching for a unique permalink (wincent.dev, 9abb465)

This prevents "foo" from being confused with "foo-bar", "foo-bar-2" and so on. I am not sure whether REGEXP is a MySQL extension or not, but seeing as I am deploying with MySQL that shouldn't be a problem anyway.

Signed-off-by: Greg Hurrell <greg@hurrell.net>