Parse the output of `svnlook diff ...`

Summary: Ref T4195. When figuring out changed content in SVN, the easiest approach is to use `svnlook diff`, but it has a slightly different header than we're used to. Adjust the parser for it and add some tests.

Test Plan: Clean unit tests.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T4195

Differential Revision: https://secure.phabricator.com/D7790
Improve `arc lint --output summary`

Summary:
This currently output like this:

  file_a:
  file_b:
  file_c:
    Warning on line 29: blah blah

This isn't especially useful and can't be piped to other tools. Instead, emit output like:

  file_c:29:Warning: blah blah

This is greppable / pipeable.

Test Plan: Ran `arc lint --output summary`.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Differential Revision: https://secure.phabricator.com/D7788