Make "bin/files" parsing of working set arguments more consistent

Summary:
Fixes T13326. In D20571, I slightly generalized construction of an iterator over a set of files, but missed some code in other "bin/files ..." commands which was also affected.

Today, basically all of these workflows define their own "--all" and "names" flags. Pull these definitions up and implement them more consistently.

Test Plan: Ran multiple different bin/files commands with different combinations of arguments, saw consistent handling of iterator construction.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13326

Differential Revision: https://secure.phabricator.com/D20614