[ci] fatal: reference is not a tree

[dev-zero]

example: https://storage.googleapis.com/cp2k-ci/run-cp2k-sdbg-cf9d8f09_report.txt

This seems to happen when the master gets an update while the CI already started the job.

At least in 2017 using the /merge ref was an undocumented feature not meant to be used by 3rd parties.

[oschuett]

Yes, this sometimes happens. It should however be fairly rare and you can always hit re-run.

Currently, I have three ideas for fixing this:

1. Perform the merge myself in run_target.sh and ci_entrypoint.sh.
2. Backup the merge branch somewhere (different repo or different branch name)
3. Add a detection and auto re-run mechanism in the backend.

I don't really like any of them.

the /merge ref was an undocumented feature not meant to be used by 3rd parties.

The merge branch is not well documented but it gets mention e.g. here. And I'm pretty sure most other CI system use it as well.