ci: checking out wrong branch?
It seems that now the default master build is always tested, not the PR build.

It looks like it was a one-time issue at #177. It was not reproduced in your PR #180. Have you seen this odd behavior anywhere else?
@felis2803 , #180 PR affected too(see actions=>Tests#45). But it's okay if I use push(--force)(actions=>Tests#46): Also #175, #179
You're right. I ran some tests in #182 and #183 and found that this behavior happens quite often. I intentionally messed up the test in #182, but it was passed.
Thank you for bringing this up.
The bug happened when GitHub couldn't create a merge commit between the PR branch and the base branch fast enough. I refactored the code that queries the merge commit SHA, so our build system should wait until the merge commit is created. It seems that the issue is fixed now.