implement mergify rules for release branches

We only handled the case of backports previously, but the current
release checklist expects that we can commit PRs to release
branches during a release (e.g. changelogs, because we want the
list of changelog.d files that are actually part of the release).

.github/mergify.yml

@@ -1,3 +1,6 @@
+# Note: We do not use the rebase strategy to merge PRs, because that
+# loses information needed by changelog-d to associate commits with PRs.
+
 pull_request_rules:
 
   # implementing PR delay logic: apply a label after 2 days of inactivity
@@ -56,7 +59,37 @@ pull_request_rules:
       - label=merge delay passed
       - '#approved-reviews-by>=2'
 
-  # rebase+merge strategy for backports: require 1 approver instead of 2
+  # merge strategy for release branches
+  - actions:
+      queue:
+        name: default
+        # Merge with a merge commit
+        method: merge
+        # Update the pr branch with rebase, so the history is clean
+        update_method: rebase
+    name: Put release branch pull requests in the rebase+merge queue
+    conditions:
+      - label=merge me
+      - base!=master
+      - -body~=backport
+      - '#approved-reviews-by>=2'
+
+  # merge+squash strategy for release branches
+  - actions:
+      queue:
+        name: default
+        method: squash
+        # both update methods get absorbed by the squash, so we use the most
+        # reliable
+        update_method: merge
+    name: Put release branch pull requests in the squash+merge queue
+    conditions:
+      - base!=master
+      - label=squash+merge me
+      - -body~=backport
+      - '#approved-reviews-by>=2'
+
+  # merge strategy for backports: require 1 approver instead of 2
   - actions:
       queue:
         name: default