fix: use commits-first approach to avoid rate limiting and missing contributors

[zkoppert]

Summary

Fixes #392 — contributors silently dropped from output due to rate limiting on large repos/orgs.

Problem

When start_date and end_date are set, get_contributors() fetches all-time contributors via repo.contributors(), then makes a separate API call per contributor to check for commits in the date range. For large repos this causes rate limiting, and the broad except Exception handler silently drops the entire repo's contributors.

Solution

Use a commits-first approach: fetch commits in the date range directly (repo.commits(since=, until=)) and extract unique authors. This reduces API calls from O(N) (all-time contributors) to O(M/30) (commits in period, paginated).

Bonus: contribution_count now reflects the actual count for the specified period, not the all-time count.

Testing

• All 62 tests pass (2 new tests added for None author handling and commit aggregation)
• Linting passes (pylint 10.00/10, flake8, mypy, black, isort all clean)
• 99.69% code coverage

[Copilot]

Pull request overview

This PR addresses missing contributors on large repos/orgs by switching get_contributors() to a commits-first strategy for date-bounded runs, reducing API calls and rate-limit risk while making counts reflect the selected period.

Changes:

• Refactors get_contributors() to enumerate commits in the start_date/end_date range and aggregate unique authors/counts.
• Updates and adds unit tests to cover commit aggregation and skipping commits with None authors.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
contributors.py	Implements commits-first contributor discovery and per-period contribution counting when a date range is provided.
test_contributors.py	Updates existing tests for the new behavior and adds new cases for None authors and aggregation across multiple commits.

Comments suppressed due to low confidence (1)

test_contributors.py:140

• test_get_contributors_skip_users_with_no_commits (and its docstring) no longer matches what the test actually verifies after the commits-first refactor; it now asserts the happy path with a single commit. Renaming the test and updating the docstring to reflect the current behavior will keep the suite understandable and avoid future confusion.

    @patch("contributors.contributor_stats.ContributorStats")
    def test_get_contributors_skip_users_with_no_commits(self, mock_contributor_stats):
        """
        Test the get_contributors function skips users with no commits in the date range.
        """
        mock_repo = MagicMock()
        mock_commit = MagicMock()
        mock_commit.author.login = "user"
        mock_commit.author.avatar_url = (
            "https://avatars.githubusercontent.com/u/12345678?v=4"
        )

        mock_repo.full_name = "owner/repo"
        mock_repo.commits.return_value = iter([mock_commit])
        ghe = ""