Deal with a situation where .config/git is a file and we notice
.config/git/config is not readable due to ENOTDIR, not ENOENT.

* jn/warn-on-inaccessible-loosen:
  config: exit on error accessing any config file
  doc: advertise GIT_CONFIG_NOSYSTEM
  config: treat user and xdg config permission problems as errors
  config, gitignore: failure to access with ENOTDIR is ok