wip: git: write-tree

Author: abhinav

internal/git/diff_wt.go renamed to internal/git/index_wt.go

@@ -76,3 +76,13 @@ func (w *Worktree) DiffIndex(ctx context.Context, treeish string) ([]FileStatus,
 
 	return files, nil
 }
+
+// WriteIndexTree writes the current index to a new tree object.
+func (w *Worktree) WriteIndexTree(ctx context.Context) (Hash, error) {
+	cmd := w.gitCmd(ctx, "write-tree")
+	out, err := cmd.OutputString(w.exec)
+	if err != nil {
+		return "", fmt.Errorf("write-tree: %w", err)
+	}
+	return Hash(out), nil
+}