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