bug#19350: #19350 24.4; Incorrect quoting of %-signs for Windows command shell

[Eli Zaretskii]

> From: npostavs@users.sourceforge.net
> Date: Sun, 14 Aug 2016 23:13:43 -0400
> Cc: 19350@debbugs.gnu.org
> 
> Hmm, maybe we could fix this by making Emacs refuse to apply environment
> variables with names ending in carets?

I'm very much against disallowing perfectly valid (if rare) use cases
just because someone malicious can take advantage of that.

>From my POV, as long as Emacs itself doesn't produce such shell
commands and/or environment variables for any of its features,
avoiding this becomes user's responsibility, just like when working at
the shell prompt.  Of course, if we can find a reliable solution to
the problem that doesn't take away features, that'd be better, but
failing that, I'm okay with leaving this alone, perhaps documenting
somewhere that using % in shell commands when there are environment
variables ending in ^ could produce surprising results.

Thanks.