[FIX] runbot_merge: point to the right status on staging failure

On staging failure, the 'bot would point to the first error or failure
status it found on the commit. This turns out not to be correct as
we (now) have various statuses which are optional, and may fail
without blocking stagings (either because they're solely informational
or because they're blocking & overridable on PRs).

Fix this so the 'bot points to the first *required* failure.

Fixes #517

runbot_merge/models/pull_requests.py

@@ -1847,12 +1847,18 @@ class Stagings(models.Model):
             if repo.endswith('^'):
                 continue
 
-            commit = self.env['runbot_merge.commit'].search([
-                ('sha', '=', head)
-            ])
+            required_statuses = set(
+                self.env['runbot_merge.repository']
+                    .search([('name', '=', repo)])
+                    .status_ids
+                    ._for_staging(self)
+                    .mapped('context'))
+
+            commit = self.env['runbot_merge.commit'].search([('sha', '=', head)])
             statuses = json.loads(commit.statuses or '{}')
             reason = next((
                 ctx for ctx, result in statuses.items()
+                if ctx in required_statuses
                 if to_status(result).get('state') in ('error', 'failure')
             ), None)
             if not reason:

runbot_merge/tests/test_basic.py

@@ -560,6 +560,7 @@ def test_staging_ci_failure_single(env, repo, users, config):
 
     staging_head = repo.commit('heads/staging.master')
     with repo:
+        repo.post_status(staging_head.id, 'failure', 'a/b')
         repo.post_status(staging_head.id, 'success', 'legal/cla')
         repo.post_status(staging_head.id, 'failure', 'ci/runbot') # stable genius
     env.run_crons()