Merge branch 'jk/config-warn-on-inaccessible-paths' into maint

Merge branch 'jk/config-warn-on-inaccessible-paths' into maint

The attribute system may be asked for a path that itself or its
leading directories no longer exists in the working tree, and it is
fine if we cannot open .gitattribute file in such a case.  Failure
to open per-directory .gitattributes with error status other than
ENOENT and ENOTDIR should be diagnosed.

* jk/config-warn-on-inaccessible-paths:
  attr: failure to open a .gitattributes file is OK with ENOTDIR
  warn_on_inaccessible(): a helper to warn on inaccessible paths
  attr: warn on inaccessible attribute files
  gitignore: report access errors of exclude files
  config: warn on inaccessible files