Fixing a crash when regenerating search results

Since Emacs does dynamic binding by default, returning this:

(lambda (mode-name) (ag/buffer-name string directory regexp))

does not evaluate `(ag/buffer-name string directory regexp)` in the
context of `ag/search`. Instead, we get an error about string being a
void variable. We force evaluation of the `ag/buffer-name` call to fix
this.