CI (Buildkite): on pull request builds, build the merge commit (e.g. refs/pull/1234/merge)

[DilumAluthge]

No description provided.

[DilumAluthge]

@staticfloat If master advances after the packager starts (but before the tester starts), the SHA of the merge commit changes. Then when the tester tries to download the artifact, it has a different filename than the file uploaded by the packager.

[DilumAluthge]

What we really want is a way to:

1. At the beginning of the build, get the SHA of the merge commit.
2. Use that same SHA for all jobs in the build, regardless of when they start.

[staticfloat]

Easiest fix; just use wildcards with buildkite-agent artifact download.

[DilumAluthge]

Easiest fix; just use wildcards with buildkite-agent artifact download.

Yeah I think this is the easiest solution. I'll also have to use wildcards a few other places, e.g. when I rename julia-COMMIT-linux64.tar.gz to julia-latest-linux64.tar.gz before uploading it to S3.

[giordano]

What happens if there is a merge conflict?

[DilumAluthge]

What happens if there is a merge conflict?

On other CI providers (e.g. Travis), CI will not start if there is a merge conflict.

For us, we can decide whether we want to:

1. Error out.
2. Run CI on the PR head branch instead.

[staticfloat]

I vote that we error out if we can't find a merge commit.

[DilumAluthge]

I'm closing this PR in favor of using the new plugin: https://github.com/JuliaCI/merge-commit-buildkite-plugin