bazel-diff: Make auto diffbase work (#1884)
Fix the logic of resolving diffbase when `USE_BAZEL_DIFF` is one of
`AUTO_DIFFBASE_VALUES`.
- Reverted the change in
https://github.com/bazelbuild/continuous-integration/pull/1883, since we
do need to download the diffbase from the original repo instead of the
PR repo.
- First determine the base branch by checking
BUILDKITE_PULL_REQUEST_BASE_BRANCH and then BUILDKITE_BRANCH
- Fetch the base branch from the original repo
- Run `git merge-base HEAD FETCH_HEAD` to calculate the diffbase commit.
1 file changed
