Merge branch 'dw/config-global-list' into seen

Merge branch 'dw/config-global-list' into seen

"git config --list --global", unlike "git config --list", did not
consult both of the two possible per-user sources of the
configuration files, i.e. $HOME/.gitconfig and the XDG one, which
has been corrected.

* dw/config-global-list:
  config: keep bailing on unreadable global files
  config: read global scope via config_sequence
  config: test home and xdg files in `list --global`
  cleanup_path: force forward slashes on Windows