Add unit tests for Git change parsers

Summary: Ref T4327. Adds some basic tests to the Git parser for a set of common operations (add, change, move, copy, directory, symlink, propchange).

Test Plan: Ran unit tests.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T4327

Differential Revision: https://secure.phabricator.com/D8012