baldrick fails to show artifacts when it's a gh-pages branch

[dpshelio]

I'm trying to show changes on a jekyll site that gh normally builds, so the repository doesn't have a master branch, only a gh-pages. In this case I'm testing this on a carpentries project that can be seen at ucl-rits/hpc-shell#1 (circle-ci output) - The error that Circle-CI gives is:
webhook returned http status 500, connecting to https://giles.cadair.dev/circleci

[Cadair]

It doesn't read the config from the HEAD to prevent people changing it in PRs to do bad things. However, this is a bug as it should read from the target branch of the PR and not master.