CommitReflogEntry now includes offset_minutes field, populated from
sig.when().offset_minutes() — matches git's internal timezone offset
representation. Used by git_tools for accurate reflog timestamps.
- Add #[serde(default)] to MergeAnalysisQuery.their_oid and MergeRefAnalysisQuery fields
since they come from path parameters, not query strings
- git_readme/git_commit_count/git_contributors: default to repo.default_branch
instead of HEAD to avoid errors when HEAD points to refs/heads/master but
the actual default branch is main
