bug#32051: 26.1; grep-regexp-alist on windows

[Eli Zaretskii]

> From: Noam Postavsky <npostavs@gmail.com>
> Date: Wed, 4 Jul 2018 15:57:53 -0400
> Cc: Dan Harms <Dan.Harms@xrtrading.com>, 32051@debbugs.gnu.org
> 
> Looking at the grep-regexp-alist, it is the case that a filename with
> a colon will not be matched when grep doesn't support --null. In the
> case where grep does support --null, the NUL byte is displayed like a
> colon, so the difference might not leap out at you.

Right, thanks.

> I think the patch below would fix it:

Looks reasonable.  Can we have a test for this use case, so that we
never again regress like that?