add ci job to find fixup commits

Using fixup commits is a helpful strategy while working on a PR. Though they
should never be merged into the `main` branch. Therefore a PR should be blocked
as long as there are still some fixup commits remaining.

Author: weilbith

.circleci/config.yml

@@ -77,6 +77,15 @@ jobs:
       - run_pre_commit_hook:
           hook_id: trailing-whitespace
 
+  find_fixup_commits:
+    executor: base_executor
+    steps:
+      - attach_workspace: **attach_options
+      - run:
+          name: Verify that there are no fixup commits in the git history
+          command: >
+            exit $(git log --pretty=format:%s main.. | grep fixup! | wc --lines)
+
 workflows:
   version: 2
   default:
@@ -88,3 +97,6 @@ workflows:
       - check_formatting:
           requires:
             - setup
+      - find_fixup_commits:
+          requires:
+            - setup