Construct a diganostic parser in BaseWorkflow::getChange()

Summary:
@ender is reporting a parsing issue in SVN, but we don't build a parser with setWriteDiffOnFailure() set in this workflow right now so I can't get the raw file to fix the issue.

Use the onboard mechanism to build a parser with `setWriteDiffOnFailure()` set, so it will write the diff, so I can get a copy so I can fix the problem.

Test Plan: Ran `arc diff --preview` in an SVN repo with a linter.

Reviewers: ender, btrahan

Reviewed By: ender

CC: aran

Differential Revision: https://secure.phabricator.com/D5699