Re: grep: fix multibyte regex handling under macOS (1819ad327b7a1f19540a819813b70a0e8a7f798f)

>
> I recently updated to git 2.39.1 and noticed today that `git diff
> --word-diff` fails for files with `diff=scheme`. I was able to narrow
> the failure down to the inclusion of control characters \xc0, \xff,
> \x80, \xbf by https://github.com/git/git/blob/2fc9e9ca3c7505bc60069f11e7ef09b1aeeee473/userdiff.c#L17
> in the definition of the scheme diff pattern (really, all patterns).
>
> I suspect the commit referenced in the subject, given that it messes
> with regex handling on macOS.
>
> Relevant environment that I can think of:
> ```
> # locale
> LANG="fr_FR.UTF-8"
> LC_COLLATE="fr_FR.UTF-8"
> LC_CTYPE="fr_FR.UTF-8"
> LC_MESSAGES="fr_FR.UTF-8"
> LC_MONETARY="fr_FR.UTF-8"
> LC_NUMERIC="fr_FR.UTF-8"
> LC_TIME="fr_FR.UTF-8"
> LC_ALL="fr_FR.UTF-8"
> ```
>
> I'm on macOS 11.7.
>
> Failure (using Zsh to produce the characters; I think there's a Bash
> equivalent):
> ```
> # git diff --word-diff --word-diff-regex=$'[\xc0-\xff][\x80-\xbf]+'
> fatal¬†: invalid regular expression: [¿-ˇ][Ä-ø]+
> ```