With default fetch flags, the if_changed logic will fail for merge bases or default branches that have not been fetched. #3602
Description
With the default flags (https://buildkite.com/docs/agent/v3/configuration#git-fetch-flags):
buildkite-agent pipeline upload .buildkite/upload-steps/unit-tests.yml
Reading pipeline configs from [".buildkite/upload-steps/unit-tests.yml"]
Updating BUILDKITE_COMMIT to "d9ad0ec59b9247d59e1f13ef2a65e8da7d05e952"
ERROR Couldn't determine git diff from upstream, not skipping any pipeline steps: git rev-parse "origin/bk-if-changed": exit status 128
ERROR git: fatal: ambiguous argument 'origin/bk-if-changed': unknown revision or path not in the working tree.
Use '--' to separate paths from revisions, like this:
'git <command> [<revision>...] -- [<file>...]'
This could be because "origin/bk-if-changed" might not be a commit in the repository.
You may need to change the --git-diff-base flag or BUILDKITE_GIT_DIFF_BASE env var.
Successfully parsed and uploaded pipeline #1 from "unit-tests.yml"
Is there a recommended set of fetch flags to use?