bug#59338: 29.0.50; Commit 1a2d603bb3 breaks Eglot on Windows

[Augusto Stoffel]

On Thu, 17 Nov 2022 at 19:06, Eli Zaretskii wrote:

> Both are wrong, right?  The correct URL should AFAIU be
>
>   file:///d:/digestif-test/tikz-test.tex
>
> IOW, the problem is that the URL is being run through url-encode-url,
> which doesn't support file:// URLs on Windows properly.

While we are at it, note that

  (url-filename
    (url-generic-parse-url "file:///d:/digestif-test/tikz-test.tex"))
  => "/d:/digestif-test/tikz-test.tex"

is not the right file name under Windows.  Eglot treats this special
case correctly, but every package that deals with file URLs has to
repeat the work.  So there should be a helper function in Emacs for
this.