CI job to check that forks are in sync (facebook#20795)

We don't always keep the reconciler forks in sync (otherwise it we
wouldn't have forked it) but during periods when they are meant to be in
sync, we use this job to confirm there are no differences.

.circleci/config.yml

@@ -376,6 +376,23 @@ jobs:
             cp ./scripts/release/ci-npmrc ~/.npmrc
             scripts/release/publish.js --ci --tags << parameters.dist_tag >>
 
+  # We don't always keep the reconciler forks in sync (otherwise it we wouldn't
+  # have forked it) but during periods when they are meant to be in sync, we
+  # use this job to confirm there are no differences.
+  sync_reconciler_forks:
+    docker: *docker
+    environment: *environment
+    steps:
+      - checkout
+      - run: yarn workspaces info | head -n -1 > workspace_info.txt
+      - *restore_node_modules
+      - run:
+          name: Confirm reconciler forks are the same
+          command: |
+            yarn replace-fork
+            git diff --quiet || (echo "Reconciler forks are not the same! Run yarn replace-fork. Or, if this was intentional, disable this CI check." && false)
+
+
 workflows:
   version: 2
   stable:
@@ -427,6 +444,11 @@ workflows:
       - yarn_flow:
           requires:
             - setup
+      # NOTE: This job is only enabled when we want the forks to be in sync.
+      # When the forks intentionally diverge, comment out the job to disable it.
+      - sync_reconciler_forks:
+          requires:
+            - setup
       - yarn_test:
           requires:
             - setup