[IMP] runbot_merge: drop merge method on squash

When moving from !squash to squash to !squash, it's a bit odd / risky
to keep the merge method as-is. So drop it.

runbot_merge/models/pull_requests.py

@@ -1055,6 +1055,9 @@ class PullRequests(models.Model):
     def write(self, vals):
         oldstate = { pr: pr._tagstate for pr in self }
 
+        if vals.get('squash'):
+            vals['merge_method'] = False
+
         w = super().write(vals)
 
         newhead = vals.get('head')

runbot_merge/tests/test_basic.py

@@ -1269,6 +1269,7 @@ class TestMergeMethod:
             ('repository.name', '=', repo.name),
             ('number', '=', prx.number),
         ])
+        pr.merge_method = 'rebase-merge'
         assert not pr.squash, "a PR with a single commit should not be squashed"
 
         with repo:
@@ -1278,6 +1279,8 @@ class TestMergeMethod:
                 force=True
             )
         assert pr.squash, "a PR with a single commit should be squashed"
+        assert not pr.merge_method, \
+            "resetting a PR to a single commit should remove the merge method"
 
     def test_pr_no_method(self, repo, env, users, config):
         """ a multi-repo PR should not be staged by default, should also get