Add git_diff_tree() for working tree diff against arbitrary revisions

[teaguesterling]

Summary

• Adds git_diff_tree() table function that compares the working tree against any git revision (not just HEAD), closing the gap identified in Working tree diff: detect modified files relative to a git revision #10
• Uses libgit2's git_diff_tree_to_workdir_with_index for accurate change detection including rename/copy tracking
• Includes LATERAL variant git_diff_tree_each() for multi-repo workflows

API

FROM git_diff_tree();                    -- workdir vs HEAD
FROM git_diff_tree('.', 'v1.0');         -- workdir vs arbitrary ref
FROM git_diff_tree(untracked:=true);     -- include untracked files

Returns: repo_path, file_path, file_ext, status (added/deleted/modified/renamed/copied/typechange), old_path

Test plan

• Zero-arg, single-arg, and two-arg overloads return correct schema (5 columns)
• Comparing against HEAD matches zero-arg default
• Comparing against older refs returns more changes
• untracked:=true includes untracked files
• Status values are constrained to expected set
• Existing test suite unaffected

Closes #10

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[teaguesterling]

Code review

No issues found. Checked for bugs and CLAUDE.md compliance.

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