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

Documentation/git-config.txt

@@ -240,6 +240,10 @@ GIT_CONFIG::
 	Using the "--global" option forces this to ~/.gitconfig. Using the
 	"--system" option forces this to $(prefix)/etc/gitconfig.
 
+GIT_CONFIG_NOSYSTEM::
+	Whether to skip reading settings from the system-wide
+	$(prefix)/etc/gitconfig file. See linkgit:git[1] for details.
+
 See also <<FILES>>.
 
 

Documentation/git.txt

@@ -772,6 +772,14 @@ for further details.
 	and read the password from its STDOUT. See also the 'core.askpass'
 	option in linkgit:git-config[1].
 
+'GIT_CONFIG_NOSYSTEM'::
+	Whether to skip reading settings from the system-wide
+	`$(prefix)/etc/gitconfig` file.  This environment variable can
+	be used along with `$HOME` and `$XDG_CONFIG_HOME` to create a
+	predictable environment for a picky script, or you can set it
+	temporarily to avoid using a buggy `/etc/gitconfig` file while
+	waiting for someone with sufficient permissions to fix it.
+
 'GIT_FLUSH'::
 	If this environment variable is set to "1", then commands such
 	as 'git blame' (in incremental mode), 'git rev-list', 'git log',