[FIX] runbot_merge: set staging_end on all terminations

Rather than only setting `staging_end` on status change, set it when
the staging gets deactivated. This way cancelling a staging (whether
explicitely or via a PR update) will also end it, so will a staging
timeout, etc..., rather than keep the counter running.

Fixes #931

runbot_merge/models/pull_requests.py

@@ -2049,6 +2049,7 @@ def write(self, vals):
                 ._trigger(fields.Datetime.to_datetime(timeout))
 
         if vals.get('active') is False:
+            vals['staging_end'] = fields.Datetime.now()
             self.env.ref("runbot_merge.staging_cron")._trigger()
 
         return super().write(vals)
@@ -2141,7 +2142,6 @@ def _compute_state(self):
             s.state = st
             if s.state != 'pending':
                 self.env.ref("runbot_merge.merge_cron")._trigger()
-                s.staging_end = fields.Datetime.now()
             if update_timeout_limit:
                 s.timeout_limit = datetime.datetime.now() + datetime.timedelta(minutes=s.target.project_id.ci_timeout)
                 self.env.ref("runbot_merge.merge_cron")._trigger(s.timeout_limit)