Please see attached patch.  There is also some early reports of
success at https://github.com/magit/magit/issues/2708

In short, I believe the analysis for this bug was thorough (and
correct!), but it turns out that the call to `git status` doesn't
actually go through the patched function so we'd end up with a few
commands getting GIT_OPTIONAL_LOCKS=0 but not the most important one.

- Andrew Ruder

P.S. still figuring out the contribution process for emacs.