[CI/CD] A more resilient github-mirror script.

- Fail immediately if no branches are checked out.

- Rebase only if we're pushing on master (don't bother for feature
  branches).

- Do a push force to Github.

.drone/github-mirror.sh

@@ -6,9 +6,18 @@
 ssh-keyscan github.com >> ~/.ssh/known_hosts 2>/dev/null
 
 # Clone the repository
+branch=$(git rev-parse --abbrev-ref HEAD)
+if [ -z "${branch}" ]; then
+  echo "No branch checked out"
+  exit 1
+fi
+
 git remote add github git@github.com:/blacklight/platypush.git
-git pull --rebase github "$(git branch | head -1 | awk '{print $2}')" || echo "No such branch on Github"
+
+if (( "$branch" == "master" )); then
+  git pull --rebase github "${branch}" || echo "No such branch on Github"
+fi
 
 # Push the changes to the GitHub mirror
-git push --all -v github
+git push -f --all -v github
 git push --tags -v github