Rebase on top of master before running build

This prevents scenarios where builds would succeed for pull requests
but failed after merging them into master, due to e.g. dependency
changes or code that looked auto mergable but wasn't.

The trade-off is that PR builds will fail faster / untimely, local
builds will however still succeed so people can rebase at their own
pace when they are ready for it.

.circleci/config.yml

@@ -1,4 +1,5 @@
 version: 2
+
 jobs:
   build:
     working_directory: /go/src/github.com/weaveworks/flux
@@ -7,6 +8,10 @@ jobs:
       - image: memcached
     steps:
       - checkout
+      # Rebase on target branch after checkout, so we test the result of a
+      # merge, and not just what is in the proposed change.
+      # Ref: https://discuss.circleci.com/t/1662
+      - run: git pull --ff-only origin "refs/pull/${CI_PULL_REQUEST//*pull\//}/merge"
       - setup_remote_docker
 
       - run: curl https://raw.githubusercontent.com/golang/dep/master/install.sh | sh
@@ -44,6 +49,11 @@ jobs:
     working_directory: ~/go/src/github.com/weaveworks/flux
     steps:
       - checkout
+      # Rebase on target branch after checkout, so we test the result of a
+      # merge, and not just what is in the proposed change.
+      # Ref: https://discuss.circleci.com/t/1662
+      - run: git pull --ff-only origin "refs/pull/${CI_PULL_REQUEST//*pull\//}/merge"
+
       - run: test/e2e/e2e-golang.sh
       - run: test/e2e/e2e-flux-build.sh
       - run: test/e2e/e2e-kind.sh