Merge branch 'jn/warn-on-inaccessible-loosen'

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

diff --cc Documentation/git-config.txt
Simple merge

diff --cc Documentation/git.txt
Simple merge

diff --cc config.c
Simple merge

diff --cc git-compat-util.h
Simple merge

diff --cc wrapper.c
Simple merge