bug#65347: 29.1; Underscore in query replace prevents case-matching

[Stefan Monnier]

>> Similarly when I say `!` it should stop after every change that
>> encountered a not-yet-seen capitalization.
>
> Define "not yet seen capitalization".

I'm replacing "foo" with "bar", so at the start we know for sure:

- "foo" should be replaced by "bar"
- that's it, anything else is guesswork

We find "Foo", it's a not-yet-seen capitalization.
We propose "Bar" (which the user may accept of modify).
At that point we know:

- "foo" should be replaced by "bar"
- "Foo" should be replaced by "Bar" (or whichever alternative the user
  provided instead).
- that's it, anything else is guesswork

When we find "fOo", it's a not-yet-seen capitalization, so we have to
ask the user again, ...


        Stefan