fix: use graph-based compare instead of timestamp when `from-tag-name` is set

[miroslavpojer]

Problem

When generating release notes for a maintenance branch (e.g. v2.6.x) that runs in parallel with a development branch (v2.7.x), the current timestamp-based commit fetch (repo.get_commits(since=…)) returns commits from all branches in the time window. This causes unrelated develop-branch PRs to appear in the maintenance patch release notes.

Solution

When from-tag-name is explicitly provided, switch to the GitHub Compare API (repo.compare(from_tag, to_tag)). This returns only commits reachable from tag-name but not from from-tag-name, regardless of timestamps or branches — identical to running:

git log --oneline --right-only --cherry-pick <from-tag>...<to-tag>

PR numbers are extracted from the returned commit messages and fetched individually. The existing timestamp path is unchanged when from-tag-name is not set.

Files affected

• release_notes_generator/model/mined_data.py — add compare_commit_shas: set[str]
• release_notes_generator/data/miner.py — add compare-mode branch and _extract_pr_numbers_from_commits helper
• release_notes_generator/data/filter.py — skip timestamp filter when compare_commit_shas is non-empty