[IMP] runbot_merge: note on behalf of which commit a staging was created

Closes #329

runbot_merge/models/pull_requests.py

@@ -436,13 +436,17 @@ class Branch(models.Model):
             r = base64.b64encode(os.urandom(12)).decode('ascii')
             trailer = ''
             if heads:
-                trailer = '\n' + '\n'.join(
+                trailer = '\n'.join(
                     'Runbot-dependency: %s:%s' % (repo, h)
                     for repo, h in heads.items()
                     if not repo.endswith('^')
                 )
             dummy_head = it['gh']('post', 'git/commits', json={
-                'message': 'force rebuild\n\nuniquifier: %s%s' % (r, trailer),
+                'message': '''force rebuild
+
+uniquifier: %s
+For-Commit-Id: %s
+%s''' % (r, it['head'], trailer),
                 'tree': tree['sha'],
                 'parents': [it['head']],
             }).json()