Managed tree diff and working-tree status (Phase B.4)

[arturcic]

Recursive tree-vs-tree changed-paths diff (for ICommit.DiffPaths) and .git/index v2-v4 reader with index-vs-HEAD and workdir-vs-index comparison for UncommittedChangesCount, matching the libgit2 adapter's status semantics including worktree-private index location.

Part of #5031 — see docs/design/managed-git-migration.md for the full design.

[arturcic]

Done via #5050 (merged into feature/managed-git).