Function resolve_ref_commit 

pub fn resolve_ref_commit(
    context: &GitContext,
    cwd: &Path,
    reference: &str,
) -> Result<Base, GitFailure>

What: Resolves a ref to its commit, or to an unborn base when it names nothing. Why: The preparation base is the target’s commit at invocation, and an unborn target starts from the empty tree.

In TS you’d write (pseudocode):

await resolveRefCommit({ gitPath, cwd, ref });