Merge branch 'mm/checkout-auto-track-fix' into maint

"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 --"