R1:3f3304fd614f

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…