pub fn stage(dir: &Path, rel_path: &str) -> Result<(), String>Expand description
Stage a path (git add -- <path>).
§Errors
Returns git’s own stderr on failure (Run H, T540 — this used to collapse
to a bare bool, so the caller could only show a static “git failed”
with no reason, inconsistent with the sibling stage_content, which
already reports the real error).