github: add mergify configuration YAML file

Start trying out mergify on sambacc.
This is highly borrowed from the go-ceph repo, which in turn highly
borrowed from the ceph-csi project's mergify configuration.
It may not be optimal but hopefully it is useful.

Signed-off-by: John Mulligan <[email protected]>

Author: phlogistonjohn

.github/mergify.yml

@@ -0,0 +1,67 @@
+---
+defaults:
+  actions:
+    queue:
+      name: default
+      method: rebase
+      update_method: rebase
+
+
+# each test should be listed separately, do not use regular expressions:
+# https://docs.mergify.io/conditions.html#validating-all-status-check
+# TODO: Use mergify's recently added 'shared configuration support'
+# to dedup some of the check-x=y repetition in the future.
+queue_rules:
+  - name: default
+    conditions:
+      - check-success=test
+      - check-success=dpulls
+
+
+pull_request_rules:
+  # Clearing approvals after content changes
+  - name: Remove outdated approvals
+    conditions:
+      - base=master
+    actions:
+      dismiss_reviews:
+        approved: true
+        changes_requested: false
+  # Perform automatic merge on conditions
+  - name: Automatic merge on approval
+    conditions:
+      - check-success=test
+      - check-success=dpulls
+      - "-draft"
+      # Contributors should set the 'do-not-merge' label if they don't want
+      # the PR to be (auto)merged for some reason.
+      - "label!=do-no-merge"
+      # A reviewer should set a label starting with 'review-in-progress' (and
+      # suffixed by their username) in order to indicate a detailed review has
+      # been started and not completed. This will hold the PR until the
+      # label has been removed.
+      - "-label~=^review-in-progress"
+      - "base=master"
+      # Even if there are 2 or more approvals we won't automerge if there are
+      # any changes requested.
+      - "#changes-requested-reviews-by=0"
+      - or:
+        # Any contributor's PR can be automerged with 2 (or more) reviews.
+        - "#approved-reviews-by>=2"
+        # A maintainer's contribution only needs 1 review BUT we give a grace
+        # period over just two weeks for a 2nd reviewer to hopefully appear.
+        - and:
+          - "updated-at<15 days ago"
+          - "author=@maintainers"
+          - "#approved-reviews-by>=1"
+    actions:
+      queue: {}
+      dismiss_reviews: {}
+  # Conflict resolution prompt
+  - name: Ask contributor to resolve a conflict
+    conditions:
+      - conflict
+    actions:
+      comment:
+        message: "This pull request now has conflicts with the target branch.
+        Please resolve these conflicts and force push the updated branch."