set tracking for current branch to github remote

Set the tracking for the current branch to the github remote, equivalent
to an initial 'push -u', e.g.:

 git config branch.master.merge  refs/heads/master
 git config branch.master.remote origin

This allows immediately doing an edit/commit and a "dzil release" with
plugins that expect to be able to just do a "git push" (e.g.
::Git::Push.)