Function materialize_pathspec_file 

pub fn materialize_pathspec_file(
    directory: &Path,
    effective_cwd: &Path,
    source: Option<&str>,
    standard_input: &mut dyn Read,
) -> Result<Option<PathBuf>>

What: Resolves a pathspec-file source to the file the private index reads. Why: A regular source is read where it is named, relative to the invocation directory, and standard input is copied once into the transaction, so no second read sees different bytes.

In TS you’d write (pseudocode):

await materializePathspecFile({ workspace, effectiveCwd, source });