[Ops] Fix scoped type check

[watson]

Problem

The .buildkite/scripts/steps/check_types_commits.sh script (introduced in #167060) is comparing the latest commit of the PR against the target branch (usually main). But if the PR is not fully up to date with the target branch, the diff is also going to contain any changes that has landed in the target branch since the PR was branched off. This is going to include a lot of extra tsconfig.json files not related to the PR and will make solving the TypeScript errors a lot harder for the PR author.

Solution

Limit diff to just the files contained in the PR by finding the closest common ancestor between the target branch and the current HEAD.

(This PR also makes the .buildkite/scripts/steps/check_types_commits.sh file executable.)

[watson]

Merged ahead of complete CI run due to this impacting a lot of developers. The modified CI steps did complete before merging

[kibana-ci]

💛 Build succeeded, but was flaky

• Buildkite Build
• Commit: 505e6c8

Failed CI Steps

• FTR Configs #27

Test Failures

• [job] [logs] FTR Configs #27 / serverless common UI Index Management Create enrich policy "before all" hook for "shows create enrich policies page and docs link"

Metrics [docs]

✅ unchanged

To update your PR or re-run it, just comment with:
@elasticmachine merge upstream

cc @watson