update_submodule: fix integration branch name in case of multiple MRs

When there are multiple open merge requests for a commit, we currently
take the newest one to determine the integration branch name. This is
not necessarily the correct one, we can have an open merge request with
the commit being part of the commit history while the top commit is a
different one.

Add a check if the commit is the top commit of the merge request in
order to get the correct integration branch name in these cases.

scripts/update_submodule.py

@@ -241,12 +241,15 @@ def update_submodule_and_include_ref(
         return None, None, None, None, None
     logging.debug("New commits: %s", commits)
 
-    # Find out if top commit is part of a merge request
+    # Find out if top commit is top commit of a merge request
     # If so, use source branch of this MR as integration branch name
     # Else use commit sha instead
     integration_branch_suffix = new_revision
     for mr in commits[0].merge_requests():
-        if mr["target_branch"] == submodule_project.default_branch:
+        if (
+            mr["target_branch"] == submodule_project.default_branch
+            and mr["sha"] == new_revision
+        ):
             integration_branch_suffix = mr["source_branch"]
             break
     logging.debug("Integration branch suffix: %s", integration_branch_suffix)