Refactor last commit retrieval to use a non deprecated method

[ludeeus]

The entire repository_object is deprecated in aiogithubapi; this is a small step in getting rid of it.

[Copilot]

Pull Request Overview

This PR refactors the code to consolidate duplicate logic for fetching repository commit information. The main change replaces direct calls to repository_object.set_last_commit() with a new method async_set_last_commits() that uses the GitHub REST API to fetch branch information.

• Introduces a new method async_set_last_commits() that fetches the last commit SHA from the repository's default branch
• Replaces two instances of duplicated commit-fetching code with calls to the new method
• Changes the implementation from using repository_object.set_last_commit() to directly calling the GitHub API

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