.gitconfig: Add default Git `push.default` config

Use the backwards-compatible `matching` setting by default. On machines with newer Git installations, it avoids the annoying Git warning, and it can be overriden to use `simple` instead in `~/.extra`.

Closes #400.
Merge remote-tracking branch 'upstream/master'

* upstream/master:
  .gitconfig: Add default Git `push.default` config