Add a simple directory fixture

Summary:
For T866, I want to write some tests which operate directly on a git repository. Provide a way to store such directories as tarballs and easily load/save them in tests.

This is similar to D2308 but much simpler since I don't yet have a good use case for class-bound fixtures. D2308 could be built on top of this sooner or later.

Test Plan: See next diff.

Reviewers: vrana, edward

Reviewed By: edward

CC: aran

Maniphest Tasks: T866

Differential Revision: https://secure.phabricator.com/D3439