D5490: commit: remove ignore whitespace option on --interactive (issue6042)

[Yuya Nishihara]

>   > `_performrevert()` would be in the same boat, but it was explicitly flagged on
>   >  at https://phab.mercurial-scm.org/rHGf37a69ec3f4717fdb4f00699ca06c225f106696c. This implies that the `diff.ignorews` option would be used
>   >  in practice to exclude whitespace changes while interactive commit/revert.
>   >  So disabling any whitespace options would break someone's workflow.
>   
>   I'm confused after this comment. How do you want me to move forward?

Yeah, I'm also confused about the current state. Honestly, I'm out of ideas.
We could add diff options to commit and revert, but I'm not sure if they
are widely used enough to being options of these commands.