feat: inspect commit history with git tree <commit> [<commit>]

[wteuber]

Summary

Adds two history-inspection modes that reuse the existing Node / NodesCollection tree renderer verbatim. git tree with no args is unchanged.

Invocation	Meaning
git tree	Working-tree status (unchanged)
git tree <commit>	Files changed in <commit> (vs. its first parent)
git tree <c1> <c2>	Files that differ between the two commits
git tree <c1>..<c2>	Same as above, as a single range token

<commit> is any revision git accepts (SHA, HEAD, HEAD~3, branch, tag).

How it works

git diff-tree --name-status -z output is translated into the porcelain entry strings the node pipeline already understands, so everything downstream is reused. Commit-mode entries render in a distinct cyan color (separate from the green=staged / red=dirty working-tree palette) with the git status letter (A/M/D/R/T); -M rename detection shows old -> new (R).

• Root commits (no parent) list every file as added (--root).
• An empty diff prints (no changes).
• Invalid revisions and the unsupported symmetric-difference A...B range fail with a clear error and non-zero exit; 3+ positional args are rejected with usage.
• -i/--indent and -c/--collapse apply in all modes; -u/--untracked-files is ignored for commit views.

Tests

• New integration tests: test_command_line_commit.rb, test_command_line_commit_range.rb (A/M/D, renames across dirs, root commit, no-changes, invalid rev, range token, reversed order, options in range mode).
• New unit test: test/git_status_tree/test_parse_diff_tree.rb (the -z token walk: regular entries, rename/copy triplets, similarity-score stripping, empty stream, multibyte paths).
• Added commit-mode cases to test_git_status_tree_class.rb, plus a teardown that resets Node's class-level state (indent/collapse_dirs/mode) so the suite is order-independent.
• ruby test/coverage.rb (the CI path): 199 runs, 0 failures, 100.00% library coverage. Verified on Ruby 2.6 (support floor) and 4.0.4.

Notes

• No version bump — changelog entry is under [Unreleased]; VERSION stays 3.6.0.
• Includes a Gemfile.lock bump (json/prism/unicode-emoji) to versions whose required_ruby_version allows Ruby 4.0 — the previously locked unicode-emoji 4.0.4 forbids Ruby 4.0 (the pinned .ruby-version), breaking bundle on 4.0.x. RuboCop stays pinned at 1.77.0 to match CI. Happy to split this out if you'd prefer the PR to be feature-only.
• Design doc included at docs/specs/git-tree-commit-args.md (tracking issue git tree [<commit>] [<commit>] #6).