Support git-format-patch format in diff parser

Summary:
Fixes T4063. The git format-patch command produces a special header
and footer which we need to detect, strip, and parse.

Test Plan:

• Added and ran unit tests.
• Submitted a diff with git format-patch HEAD^ --stdout | arc diff --raw.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T4063

Differential Revision: https://secure.phabricator.com/D8547