After pushing, wait for MRs to stabilize

By stabililze we mean that the "changes_count" field of the MR has a
value of "1".  This ensures that by the time the "git lab" command
terminates, MRs should be fully up-to-date.

Note that this approach will have to be reevaluated if #16 is
implemented (which would allow multiple commits to end up in a single
MR).

Change-Id: Id20d9feb27076afe0603bee334badc815e5528a9

gerritlab/main.py

@@ -173,6 +173,10 @@ def cancel_prev_pipelines(commit):
     with timing("push"):
         remote.push(refspec=refs_to_push, force=True)        
 
+    with timing("stabilize"):
+        for mr in new_mrs+updated_mrs:
+            mr.wait_until_stable()
+
     if len(updated_mrs) == 0 and len(new_mrs) == 0:
         print()
         warn("No updated/new MRs.\n")

gerritlab/merge_request.py

@@ -33,6 +33,9 @@ def __init__(
         self._web_url = None
         self._mergeable = False
 
+        self._set_data(json_data)
+
+    def _set_data(self, json_data):
         if json_data is not None:
             for attr in json_data:
                 setattr(self, "_{}".format(attr), json_data[attr])
@@ -157,6 +160,25 @@ def needs_update(self, commit) -> bool:
                 self._title != title or 
                 self._description != desc.strip())
 
+    def refresh(self):
+        """
+        Update's this object's data using the latest info available from the server.
+        """
+        r = global_vars.session.get("{}/{}".format(global_vars.mr_url, self._iid))
+        r.raise_for_status()
+        self._set_data(r.json())
+
+    def wait_until_stable(self):
+        """
+        Poll GitLab until the changes_count field has a value of "1".
+        Other values indicate that GitLab hasn't reacted to a push yet.
+        """
+        while True:
+            self.refresh()
+            if self._changes_count == "1":
+                return
+            time.sleep(0.500)
+
 
 def _get_open_merge_requests():
     """Gets all open merge requests in the GitLab repo."""