Fixed 'Create Diff'

Summary:
'Create Diff' with whitespace mode 'ignore-all' is borken, because the
line numbers get mixed up when creating a second diff for the
whitespace changes.

This should be fixed correctly at some point, but currently the
whitespace 'ignore-all' says 'Huge mess' in the comments and I didn't
want to make the mess any bigger.

Test Plan:
Tested that 'Create Diff' showed the diff correctly and a previously
created diff looked correct once the cache was disabled.

Reviewed By: epriestley
Reviewers: epriestley, jungejason
CC: aran, epriestley
Differential Revision: 473