Rename a constant

Summary: Rename DifferentialChangesetParser::WHITESPACE_IGNORE_FORCE to DifferentialChangesetParser::WHITESPACE_IGNORE_ALL to better reflect reality.

Test Plan: Viewed a diff with various settings for the "Whitespace changes" option.

Reviewers: #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: epriestley

Differential Revision: https://secure.phabricator.com/D11730