Implement GIT_CONFIG_NOGLOBAL
Right now, to ensure that git is running without any system- or user-provided configuration, we'd have to rune git with GIT_CONFIG_NOSYSTEM=true, HOME=/dev/null, XDG_CONFIG_HOME=/dev/null. Modifying the HOME variables is awkward, so we should instead implement a GIT_CONFIG_NOGLOBAL to disable loading user configuration.