"git checkout topic", when there is not yet a local "topic" branch
but there is a unique remote-tracking branch for a remote "topic"
branch, pretended as if "git checkout -t -b topic remote/$r/topic"
(for that unique remote $r) was run. This hack however was not
implemented for "git checkout topic --".
* mm/checkout-auto-track-fix:
checkout: proper error message on 'git checkout foo bar --'
checkout: allow dwim for branch creation for "git checkout $branch --"