fix: make git integration robust to renames, non-ASCII paths, and cwd

[KEY60228]

Summary

Reworks the git module around vim.system() argv calls executed with git -C <root>, fixing five failure modes found in a full-project review:

• Renamed/copied files: --name-status rename lines (R100\told\tnew) were parsed as a single bogus path with status "R100", breaking diff retrieval, the file tree, and Markdown output. Now parsed via NUL-separated -z output; rename diffs pass both paths as pathspec so git pairs them, and Markdown shows Renamed from <old>.
• Non-ASCII filenames (e.g. Japanese): with default core.quotepath, paths were stored as octal-escaped quoted strings that matched nothing. -z output is unquoted raw bytes, so they now work regardless of the setting.
• cwd independence: all commands run with git -C <root>, so per-file pathspecs resolve correctly when Neovim is started in a subdirectory. Previously the empty diff silently triggered the --no-index fallback and displayed tracked files as full additions.
• Staged changes: no-argument :ReviewThemStart now diffs HEAD against the working tree (falling back to the index diff in unborn repos), so staged edits are reviewable as the docs describe.
• --no-index fallback: only applied to files explicitly flagged as untracked by ls-files, instead of firing whenever a diff came back empty.

get_file_diff now takes the DiffFile entry (bare paths still accepted for compatibility). Docs updated to describe the merge-base semantics of the base-only form and the staged/untracked coverage of the no-argument form.

Test plan

Verified with a headless-nvim script against a fixture repo (rename commit, Japanese filenames, staged + untracked files, run from a subdirectory): all 13 checks pass, including rename parse (status=R, old_path set), unquoted Japanese paths, staged file inclusion, per-file hunks from a subdirectory, no --no-index misfire for tracked files, and end-to-end session.create.

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