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