zsh_completion: declare appropriate local parameters for ->string form
When ->string form is used for _arguments, the function that calls it must
declare appropriate local parameters.
Managing local return value is needed to tell the completion system if our
function succeeded in suggesting something or not, plus without that `hg diff
-<TAB>` doesn't look right.
While at it, fix a copypaste error (s/diff_files/revert_files/).
Differential Revision: https://phab.mercurial-scm.org/D4264