Differential whitespace mode IGNORE ALL now shows correct indentation

Summary:
Fixed buggy and incomplete logic for handling IGNORE ALL mode properly.
A subparser is used to parse the non-ws-ignoring changeset while a
ws-ignoring changeset is handed off to the original parser. At a later
step, the original parser queries the subparser for its lines of text
(which are formatted properly due to being in non-ws-ignoring mode) and
uses them to replace the text in the ws-ignoring diff.

Task ID: 549940

Test Plan:
-turn off caching temporarily (the cached view is still indented
improperly)
-visit http://phabricator.dev1943.facebook.com/D242591
-note aligned, but not completely highlighted, indentation right above the
comments complaining about
indentation issues

Reviewed By: aran
Reviewers: tuomaspelkonen, jungejason, aran
Commenters: epriestley
CC: aran, epriestley, grglr
Differential Revision: 174