Function classify_diff_arg 

pub fn classify_diff_arg(arg: &str, repo_root: &Path) -> Result<DiffArg, Error>

Classify a diff argument as a snapshot file or git ref.

Detection order:

1. Existing file on disk -> Snapshot
2. git rev-parse --verify <arg>^{commit} succeeds -> GitRef
3. Path-like arg that doesn’t exist (contains / or .json) -> error (file not found)
4. Neither -> error