[FIX] runbot_merge: only update pending staging state

The staging validation routine would ignore stagings which were
cancelled or ff_failed, but it should also have ignored failed and
successful aka all terminal state.

Simplify the condition for that: just ignore a staging's validation if
the staging is not pending.

Closes #211

runbot_merge/models/pull_requests.py

@@ -1311,7 +1311,7 @@ class Stagings(models.Model):
         ('pending', 'Pending'),
         ('cancelled', "Cancelled"),
         ('ff_failed', "Fast forward failed")
-    ])
+    ], default='pending')
     active = fields.Boolean(default=True)
 
     staged_at = fields.Datetime(default=fields.Datetime.now)
@@ -1362,7 +1362,7 @@ class Stagings(models.Model):
     def _validate(self):
         Commits = self.env['runbot_merge.commit']
         for s in self:
-            if s.state in ('cancelled', 'ff_failed'):
+            if s.state != 'pending':
                 continue
 
             heads = [