Create a fresh staging branch from the latest deployed master.

create_staging_branch.sh

#!/bin/bash
#
# Assumptions:
# - Clean working tree.
# - Tags are used exclusively to tag deploys

git checkout master
git branch -D staging # Delete staging branch locally.
git push origin :staging # Delete staging branch from GitHub.
git checkout -b staging `git describe --abbrev=0 --tags` # create staging from the latest deployed tag.
git push --set-upstream origin staging