fix(ci): unshallow repo before merge in post-checkout hook

[ycombinator]

Problem

Follow-up to #7029, which added the sparse-checkout#v1.6.0 Buildkite plugin to catalog-info.yaml. Once merged, Buildkite updated its pipeline configuration globally, so all builds — including PRs targeting non-main branches — now use sparse checkout.

The sparse clone fetches only ~180 git objects (the .buildkite and .go-version file trees), with no commit-ancestry chain. When .buildkite/hooks/post-checkout then fetches the target branch fully and calls git merge "${BUILDKITE_COMMIT}", git cannot walk back through the truncated history to find the common ancestor and fails with:

fatal: refusing to merge unrelated histories

This broke all open PRs targeting non-main branches (e.g. backport PRs to 8.19 such as #7036, #7030).

Fix

Unshallow the repository before attempting the merge, so git can resolve the merge base correctly. If --unshallow fails, the clone is not shallow and full history is already present, so we just continue.

Test plan

• Verify that PRs targeting 8.19 (e.g. [8.19](backport #7026) build(deps): bump the elastic-apm group across 1 directory with 6 updates #7036, [updatecli] update elastic stack version for testing 8.19.16-9ddb044b-SNAPSHOT #7030) pass the Buildkite post-checkout step after this is merged and the same fix is backported to 8.19

🤖 Generated with Claude Code

[mergify]

This pull request does not have a backport label. Could you fix it @ycombinator? 🙏
To fixup this pull request, you need to add the backport labels for the needed
branches, such as:

• backport-./d./d is the label to automatically backport to the 8./d branch. /d is the digit
• backport-active-all is the label that automatically backports to all active branches.
• backport-active-8 is the label that automatically backports to all active minor branches for the 8 major.
• backport-active-9 is the label that automatically backports to all active minor branches for the 9 major.