bug#43577: wrong result for grep -io in turkish locale

[Paul Eggert]

On 9/23/20 7:30 AM, Jim Meyering wrote:
$ LC_ALL=tr_TR.utf8 src/grep -i a
zsh: abort (core dumped)  LC_ALL=tr_TR.utf8 src/grep -i a

I can reproduce this bug. There seems to be a performance regression too. I'll 
look into it.