Merge branch 'cn/branch-set-upstream-to'

"git branch --set-upstream origin/master" is a common mistake to create a local branch 'origin/master' and set it to integrate with the current branch. With a plan to deprecate this option, introduce "git branch (-u|--set-upstream-to) origin/master" that sets the current branch to integrate with 'origin/master' remote tracking branch. * cn/branch-set-upstream-to: branch: deprecate --set-upstream and show help if we detect possible mistaken use branch: add --unset-upstream option branch: introduce --set-upstream-to