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