Strip comments from all diffs before parsing them

Summary:
See <https://github.com/facebook/phabricator/issues/400>. Since all of `patch`, `git apply` and `hg export` either accept or emit header comments, parse them unconditionally.

This is a tiny bit messy because we already had a less-general parser for `hg export` diffs, which have a large header section.

This is less permissive than GNU `patch`, which allows comments anywhere. We could do that, but `git apply` won't read them and they seem pretty crazy.

Test Plan: Added and ran unit tests.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Differential Revision: https://secure.phabricator.com/D7207