Fix two parsing issues for diffs with damaged whitespace

Summary:
Fixes T4941. If a diff has had trailing whitespace stripped, we will fail to handle empty lines correctly (previously, these lines had a leading space when the original tool emitted them).

(This probably stopped working around the time we began retaining newlines.)

Test Plan: The diff in T4941 now parses and renders correctly.

Reviewers: asherkin, btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T4941

Differential Revision: https://secure.phabricator.com/D8968