I've been struggling with these two variables being nil on my build, and
finally I modified emacs-repository-version-git to write a log file and see
what was happening:

fatal: detected dubious ownership in repository at
'D:/Devel/emacs/repo/trunk'
'D:/Devel/emacs/repo/trunk/.git' is owned by:
        'S-1-5-32-544'
but the current user is:
        'S-1-5-21-...' [my account's SID]
To add an exception for this directory, call:

        git config --global --add safe.directory D:/Devel/emacs/repo/trunk

One of these cases where there's a directory whose owner is the
Administrators group. Changing the ownership of .git/ to the current user
fixed the problem.

BTW, the git config line didn't help.

Is this usual? Should it be mentioned on some README? (Or perhaps it is
already and I did miss it.)