Function initialize_commit_index 

pub fn initialize_commit_index(
    context: &GitContext,
    cwd: &Path,
    real_index: &Path,
    indexes: &CaptureIndexes,
    selection: &IndexSelection<'_>,
) -> Result<(), GitFailure>

What: Copies or builds the private commit index and snapshots the real index at invocation. Why: Index commits start from the real index, so their staged state is the state at invocation; explicit-path commits start from the recorded base, never live HEAD, because another landing may have moved it since invocation.

In TS you’d write (pseudocode):

await initializeCommitIndex({ workspace, gitPath, cwd, mode, pathspecs, baseRevision, ... });