ci: check if commit message contains fixup on travis

.travis.yml

@@ -33,6 +33,7 @@ script:
     - pipenv run make style_check
     - pipenv run make defs_check
     - pipenv run make gen_check
+    - ci/check_fixup.sh
 
 notifications:
     webhooks:

ci/check_fixup.sh

@@ -0,0 +1,12 @@
+#!/usr/bin/env bash
+
+git fetch origin master:master  # on CIs it is common to fetch only the current branch
+
+messages=$(git log --format=%s master..)  # commit messages between this branch and master
+
+if [[ $messages =~ "fixup!" ]]; then
+  echo 'Failure: Some commit message contains "fixup!" string.'
+  exit 1
+else
+  echo 'Success: No commit messages containing "fixup!" string.'
+fi