ci: test merge commits on circle

We expect this behavior because it's what Travis does. Also it's better because we want
to test what happens if we merge the PR, not the status of the PR branch.

.circleci/config.yml

@@ -5,7 +5,11 @@ jobs:
     docker:
       - image: angular/ngcontainer
     steps:
-      - checkout
+      - checkout:
+          # After checkout, rebase on top of master.
+          # Similar to travis behavior, but not quite the same.
+          # See https://discuss.circleci.com/t/1662
+          post: git pull --ff-only origin "refs/pull/${CI_PULL_REQUEST//*pull\//}/merge"
       - restore_cache:
           key: angular-{{ .Branch }}-{{ checksum "npm-shrinkwrap.json" }}
       - run: npm install