diff: underline added/removed parts by default

[jonathantanmy]

This helps better visualize changes that contain leading and/or trailing whitespace.

It looks like this: (hmm...maybe the underlines would be confused with underscores)

Checklist

If applicable:

• [ ] I have updated CHANGELOG.md
• [ ] I have updated the documentation (README.md, docs/, demos/)
• [ ] I have updated the config schema (cli/src/config-schema.json)
• [ ] I have added tests to cover my changes

[jonathantanmy]

Thanks @PhilipMetzger .

[emilazy]

I like this (I currently pass --git all the time because the default diff format output feels too ambiguous, esp. with regards to whitespace). The underscore ambiguity seems limited; any added or removed underscore would also receive the underline, so I imagine users would catch on quickly. I guess some terminal emulators and fonts might draw the underline at the same place as an underscore?

[yuja]

Just to be sure, this also affects diff --git output.

[martinvonz]

Just to be sure, this also affects diff --git output.

Oh, good catch! I don't think we want that.

[martinvonz]

3965 should fix