Merge branch 'kh/maintenance-use-xdg-when-it-should'

When $HOME/.gitignore is missing but XDG config file available, we
should write into the latter, not former.  "git gc" and "git
maintenance" wrote into a wrong "global config" file, which have
been corrected.

* kh/maintenance-use-xdg-when-it-should:
  maintenance: use XDG config if it exists
  config: factor out global config file retrieval
  config: rename global config function
  config: format newlines