[CHEF-3940] ensure $HOME matches user executing `git`

Certain versions of `git` misbehave if git configuration is
inaccessible in $HOME. We need to ensure $HOME matches the
user who is executing `git` not the user running Chef.
CHEF-3940: Raise an error if we cannot find the user

We need the user to find their HOME, and we don't want to guess about the HOME.