R1:3f8eccdaec8f

Put some whitespace behaviors back, but only for "diff alignment", not display

Summary:
Depends on D20185. Ref T13161. Fixes T6791.

See some discusison in T13161. I want to move to a world where:

- whitespace changes are always shown, so users writing YAML and Python are happy without adjusting settings;
- the visual impact of indentation-only whitespace changes is significanlty reduced, so indentation changes are easy to read and users writing Javascript or other flavors of Javascript are happy.