add squash+merge strategy for backports

.github/mergify.yml

@@ -71,6 +71,21 @@ pull_request_rules:
  - body~=backport
  - '#approved-reviews-by>=1'
 
+ # merge+squash strategy for backports: require 1 approver instead of 2
+ - 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 backports in the squash+merge queue
+ conditions:
+ - label=squash+merge me
+ - base!=master
+ - body~=backport
+ - '#approved-reviews-by>=1'
+
  # backports should be labeled as such
  - actions:
  label: