[IMP] runbot_merge: quickfix to mark PRs as merging

Apparently the split and move arounds caused the _tagstate to get
computed/updated earlier (/differently), and thus the tagging update
relying on it to... not work anymore.

At least restore adding a "merging" tag when a PR is staged.

runbot_merge/models/pull_requests.py

@@ -356,7 +356,7 @@ class Branch(models.Model):
             Tagging.create({
                 'pull_request': pr.number,
                 'repository': pr.repository.id,
-                'state_from': pr._tagstate,
+                'state_from': 'ready',
                 'state_to': 'staged',
             })