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

When looking for $HOME/.gitconfig etc., it is OK if we cannot read
them because they do not exist, but we did not diagnose existing
files that we cannot read.

* jk/config-warn-on-inaccessible-paths:
  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

diff --cc builtin/config.c
Simple merge

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