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

When attempting to read the XDG-style $HOME/.config/git/config and
finding that $HOME/.config/git is a file, we gave a wrong error
message, instead of treating the case as "a custom config file does
not exist there" and moving on.

* 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

git-compat-util.h

@@ -637,8 +637,12 @@ int rmdir_or_warn(const char *path);
  */
 int remove_or_warn(unsigned int mode, const char *path);
 
-/* Call access(2), but warn for any error besides ENOENT. */
+/*
+ * Call access(2), but warn for any error except "missing file"
+ * (ENOENT or ENOTDIR).
+ */
 int access_or_warn(const char *path, int mode);
+int access_or_die(const char *path, int mode);
 
 /* Warn on an inaccessible file that ought to be accessible */
 void warn_on_inaccessible(const char *path);