Function scan_commit 

pub async fn scan_commit(cwd: &Path, sha: &str) -> CommitContent

Read one commit’s own diff as bounded content, under its own budget.

The ref spec is super::hunks::scan_commit’s, so the two reads describe the same range; the caps are this read’s, spent fresh because the answer stands alone rather than sharing a record’s one budget. Clean for a diff with nothing in it, which for a commit is an empty change rather than a missing one.