Merge branch 'yn/git-jump-emacs'

"git jump" (in contrib/) learned to present the "quickfix list" to
its standard output (instead of letting it consumed by the editor
it invokes), and learned to also drive emacs/emacsclient.

* yn/git-jump-emacs:
  git-jump: invoke emacs/emacsclient
  git-jump: move valid-mode check earlier
  git-jump: add an optional argument '--stdout'