Add get_merge_base to ddev git

[lucia-sb]

What does this PR do?

Adds a new helper method get_merge_base in the GitRepository class in ddev. It returns the commit hash of the common ancestor between HEAD and the given reference.

Motivation

The quality gates need to get the merge base in order to compare sizes in PRs.

Review checklist (to be filled by reviewers)

• Feature or bugfix MUST have appropriate tests (unit, integration, e2e)
• Add the qa/skip-qa label if the PR doesn't need to be tested during QA.
• If you need to backport this PR to another branch, you can add the backport/<branch-name> label to the PR and it will automatically open a backport PR once this one is merged

[github-actions]

⚠️ Recommendation: Add qa/skip-qa Label

This PR does not modify any files shipped with the agent.

To help streamline the release process, please consider adding the qa/skip-qa label if these changes do not require QA testing.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.01%. Comparing base (d39952e) to head (bd0c626).
⚠️ Report is 6 commits behind head on master.

Additional details and impacted files

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[AAraKKe]

🚀