Make directory fixture script slightly more modern and usable

Summary:
Ref T4327. I made some small changes to this script while developing parser unit tests. Mostly:

• Add pht().
• Use PhutilConsole.
• Throw PhutilArugmentUsageException for questionable usage.
• Add a --read-only flag to ignore changes on exit.
• Show the CWD after unpacking the fixture, which makes it easier to svn checkout file://... it.
• Improve some language.

Test Plan: Tested all the features; used these features while working on T4327 to good effect.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T4327

Differential Revision: https://secure.phabricator.com/D8021