Complete against a list of directory candidates.
If REGEXP is non-nil, it is a regular expression used to refine the
match (files not matching the REGEXP will be excluded).
If PREDICATE is non-nil, it will also be used to refine the match
(files for which the PREDICATE returns nil will be excluded).
If no directory information can be extracted from the completed
component, `default-directory' is used as the basis for completion.
(fn &optional REGEXP PREDICATE)