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

Finishing touches to the recently graduated topic to introduce
"git branch --set-upstream-to" option.

* cn/branch-set-upstream-to:
  completion: complete branch name for "branch --set-upstream-to="
  completion: add --set-upstream-to and --unset-upstream

diff --cc contrib/completion/git-completion.bash
Simple merge