The cron job that runs every day would fail because git checkout a single commit, not a branch.
#12708 introduce a method to check whether merges are clean. However, there are four merges are not clean. So, I add a list of merges that are dirty and ignore them.
Test: https://travis-ci.org/ken2812221/bitcoin/jobs/370356336
Edit: Seems the maximum job running time is 50 mins, so should we limit the count of clean merge tests?