Use 'git blame --porcelain' for git blame info

Summary:
This guards against stability issues with the output format of 'git
blame' (such as git config, localization (ref T5554) or future changes).

For example, git config blame.blankboundary true breaks arc cover
before this patch.

Test Plan:

• Set git config blame.blankboundary true on a test repo.
• Ran arc cover. It failed with an exception ("Bad blame?").
• Applied this patch.
• arc cover works.

Reviewers: #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: Korvin

Maniphest Tasks: T5554

Differential Revision: https://secure.phabricator.com/D13993