blackbox: do not nullify repo to deactivate the logger on failure

The _repo will be a mandatory attribute. Instead, make the logger to not
track any events.

hgext/blackbox.py

@@ -184,7 +184,7 b' class blackboxlogger(object):'
                 fp.write(fmt % args)
         except (IOError, OSError) as err:
             # deactivate this to avoid failed logging again
-            self._repo = None
+            self._trackedevents.clear()
             ui.debug('warning: cannot write to blackbox.log: %s\n' %
                      encoding.strtolocal(err.strerror))