Use git push -f when pushing into backport branches (#32)

This solves issues with stale remote branches without having to keep track of
them explicitly.

Co-authored-by: Pablo Galindo Salgado <Pablogsal@gmail.com>

Author: ambv

cherry_picker/cherry_picker.py

@@ -284,7 +284,11 @@ def push_to_remote(self, base_branch, head_branch, commit_message=""):
         """ git push <origin> <branchname> """
         set_state(WORKFLOW_STATES.PUSHING_TO_REMOTE)
 
-        cmd = ["git", "push", self.pr_remote, f"{head_branch}:{head_branch}"]
+        cmd = ["git", "push"]
+        if head_branch.startswith("backport-"):
+            # Overwrite potential stale backport branches with extreme prejudice.
+            cmd.append("--force-with-lease")
+        cmd += [self.pr_remote, f"{head_branch}:{head_branch}"]
         try:
             self.run_cmd(cmd)
             set_state(WORKFLOW_STATES.PUSHED_TO_REMOTE)