events: use branch from previous commit for repo push event commits

data so that grouping works, fixes  #4233

rhodecode/events/repo.py

@@ -237,6 +237,11 b' class RepoPushEvent(RepoVCSEvent):'
 
         commits = _commits_as_dict(
             commit_ids=self.pushed_commit_ids, repos=[self.repo])
+
+        last_branch = None
+        for commit in reversed(commits):
+            commit['branch'] = commit['branch'] or last_branch
+            last_branch = commit['branch']
         issues = _issues_as_dict(commits)
 
         branches = set(