get_commits() support

[maciekbanas]

Part of #754.

Implement get_commits() for BitBucket Cloud:

• Retrieve commits via GET /2.0/repositories/{workspace}/{repo_slug}/commits.
• BitBucket does not support date-range filtering on the commits endpoint — implement client-side filtering by paginating and stopping when commits fall outside the since window.
• Map commit data to the standard gitstats_commits table: id, committed_date, author, author_login, author_name, additions, deletions, repo_name, organization, repo_url, api_url.
• Note: BitBucket commit responses don't include additions/deletions inline (requires a separate diffstat call). Initially set to NA; a follow-up can add diffstat support.
• Unit tests with mocked API responses.