[FIX] runbot_merge: incorrect attribute access

Seems to be a pretty long-standing issue but I'd not noticed it before
as it's rather rarely taken & our sentry remains rather blown to hell,
I only happened to stumble upon the issue in the logs.

There's no ``number`` attribute on the repository object (to which
``_load_pr`` belongs). We obviously want to use the number of the PR
we're currently loading.

runbot_merge/models/pull_requests.py

@@ -309,7 +309,7 @@ All substitutions are tentatively applied sequentially to the input.
             })
             self.env['runbot_merge.pull_requests.feedback'].create({
                 'repository': pr_id.repository.id,
-                'pull_request': self.number,
+                'pull_request': number,
                 'message': r,
             })
             return