Pull request builds checkout wrong commits

https://travis-ci.com/atlassian/jira-actions/jobs/201882112

This build says it was building https://github.com/atlassian/jira-actions/commit/81a7f9779cfaf402379741bb9d2370683559014f but what it did in reality was:

git fetch upstream +refs/pull/11/merge
git checkout -qf FETCH_HEAD

This is not the same commit! It’s a totally different file tree. The PR has moved ahead between triggering the build and the actual checkout.
Travis is making red-green-refactor TDD methodologies a pain already. When I need to show that a build is red and a subsequent commit is green, I have to push the commits one by one. With this bug in place I also have to wait for the checkout to happen between the pushes! This is extra counterproductive when the builds get queued.

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