Fix a TODO
Summary: Rename `DifferentialChangesetParser::WHITESPACE_IGNORE_ALL` to `DifferentialChangesetParser::WHITESPACE_IGNORE_MOST`.
Test Plan: Browsed a diff with a few different settings for "Whitespace changes".
Reviewers: epriestley, #blessed_reviewers
Reviewed By: epriestley, #blessed_reviewers
Subscribers: epriestley
Differential Revision: https://secure.phabricator.com/D11715