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]+

(Looks like the output is a bit scrambled; here's the hexdump)

# !! |& xxd
00000000: 6661 7461 6cc2 a03a 2069 6e76 616c 6964  fatal..: invalid
00000010: 2072 6567 756c 6172 2065 7870 7265 7373   regular express
00000020: 696f 6e3a 205b c02d ff5d 5b80 2dbf 5d2b  ion: [.-.][.-.]+
00000030: 0a                                       .

-- 
D. Ben Knoble