bug#23769: 25.0.95; Mode Line breakage in vc-git

[Eli Zaretskii]

> Cc: 23769@debbugs.gnu.org, phillip.lord@russet.org.uk
> From: Dmitry Gutov <dgutov@yandex.ru>
> Date: Fri, 17 Jun 2016 15:06:10 +0300
> 
> On 06/17/2016 09:54 AM, Eli Zaretskii wrote:
> 
> > (setenv "FOO") works for me.
> 
> setenv works destructively. To use it and remain good citizens, we'd 
> have to `(copy-sequence process-environment)` every time we call a Git 
> command.
> 
> Maybe that's not huge in the grand scheme of things, but it certainly 
> looks wasteful.

If you want to manually remove the variable before adding it with no
value, that will also work.  Otherwise, I see no magic that would do
anything beyond the expected effect on any list.