[FIX] runbot_merge: mistake in logging parameter

When it updated tagging e82de3136b also
incorrectly replaced a `pr` by `pr.display_name`, probably leftover
from an attempt to update a callsite from `str(pr)` to
`pr.display_name` which I missed when reverting that.

Anyway at that section, `pr` is an integer (as it comes from an SQL
query) not an object.

runbot_merge/models/pull_requests.py

@@ -118,7 +118,7 @@ class Project(models.Model):
             except Exception:
                 _logger.exception(
                     "Error while trying to change the tags of %s#%s from %s to %s",
-                    repo.name, pr.display_name, remove, add,
+                    repo.name, pr, remove, add,
                 )
             else:
                 to_remove.extend(ids)