R1:fe7047d12d69

Display some invisible/nonprintable characters in diffs by default

Summary:
Ref T12822. Ref T2495. This is the good version of D20193.

Currently, we display various nonprintable characters (ZWS, nonbreaking space, various control characters) as themselves, so they're generally invisible.

In T12822, one user reports that all their engineers frequently type ZWS characters into source somehow? I don't really believe this (??), and this should be fixed in lint.

That said, the only real reason not to show…