Update existing backports in the backport script

Depending on the branch protection settings, we might have to update
them before they can be merged, so do this automatically to minimize the
required manual work.

scripts/backport.py

@@ -411,19 +411,33 @@ def report_backport_not_done(original_pr, reason, details=None):
     backport_branch = f"backport/{backport_target}/{original_pr.number}"
 
     # If there is already a backport branch for this PR, this probably means
-    # that we already created the backport PR. Skip it.
+    # that we already created the backport PR. Update it, because the PR might
+    # not auto-merge when the branch is not up to date with the target branch,
+    # depending on the branch protection settings. We want to update to the
+    # recent target automatically to minimize the amount of manual work.
     if (
         git_returncode(f"rev-parse {target_remote}/{backport_branch} > /dev/null 2>&1")
         == 0
     ):
         print(
-            f'Backport branch {backport_branch} for PR #{original_pr.number}: "{original_pr.title}" already exists. Skipping.'
+            f'Backport branch {backport_branch} for PR #{original_pr.number}: "{original_pr.title}" already exists. Updating.'
         )
+        git_check("reset --hard")
+        git_check("clean -xfd")
+        git_check(
+            f"checkout --quiet --force --detach {target_remote}/{backport_branch} > /dev/null"
+        )
+        # Use merge and no force-push, so that the simultaneous changes made by
+        # other users are not accidentally overwritten.
+        git_check(f"merge --quiet {target_remote}/{backport_target}")
+        git_check(f"push --quiet {target_remote} @:{backport_branch}")
         continue
 
     # Try to cherry-pick the commits.
+    git_check("reset --hard")
+    git_check("clean -xfd")
     git_check(
-        f"checkout --quiet --detach {target_remote}/{backport_target} > /dev/null"
+        f"checkout --quiet --force --detach {target_remote}/{backport_target} > /dev/null"
     )
 
     commit_shas = [commit.sha for commit in pr_info.pygithub_commits]