[FIX] runbot_merge: FF error logging message

In case of error while fast-forwarding a staging to its source, we'd
log the target to which we couldn't FF. Sadly this relied on a
`repo_name` variable which (likely since the introduction of the
"safety dance" fast forwarding) can not actually be set in case of
failure.

So stash the relevant bit (the repo name) inside the FF error exception
and use that to compose our logging message instead of a variable which
can only be None.

runbot_merge/github.py

@@ -116,7 +116,7 @@ class GH(object):
             _logger.debug('fast_forward(%s, %s, %s) -> OK', self._repo, branch, sha)
         except requests.HTTPError:
             _logger.debug('fast_forward(%s, %s, %s) -> ERROR', self._repo, branch, sha, exc_info=True)
-            raise exceptions.FastForwardError()
+            raise exceptions.FastForwardError(self._repo)
 
     def set_ref(self, branch, sha):
         # force-update ref

runbot_merge/models/pull_requests.py

@@ -1402,7 +1402,6 @@ class Stagings(models.Model):
         project = self.target.project_id
         if self.state == 'success':
             gh = {repo.name: repo.github() for repo in project.repo_ids}
-            repo_name = None
             staging_heads = json.loads(self.heads)
             self.env.cr.execute('''
             SELECT 1 FROM runbot_merge_pull_requests
@@ -1410,11 +1409,11 @@ class Stagings(models.Model):
             FOR UPDATE
             ''', [tuple(self.mapped('batch_ids.prs.id'))])
             try:
-                repo_name = self._safety_dance(gh, staging_heads)
+                self._safety_dance(gh, staging_heads)
             except exceptions.FastForwardError as e:
                 logger.warning(
                     "Could not fast-forward successful staging on %s:%s",
-                    repo_name, self.target.name,
+                    e.args[0], self.target.name,
                     exc_info=True
                 )
                 self.write({