Fix reuse of locals as iterators in libphutil

Summary:
Only one hit here. It did not cause a bug, but is unconventional. Use a more conventional construction.

Also added some test coverage.

Test Plan: Ran tests.

Reviewers: vrana, btrahan

Reviewed By: vrana

CC: aran, epriestley

Differential Revision: https://secure.phabricator.com/D2051