Fix merging child stack into parent (#12426)

# Description
Fixes #12423 where changes to mutable variables are not properly
persisted after a REPL entry.

tests/shell/repl.rs

@@ -0,0 +1,9 @@
+use nu_test_support::{nu, nu_repl_code};
+use pretty_assertions::assert_eq;
+
+#[test]
+fn mut_variable() {
+    let lines = &["mut x = 0", "$x = 1", "$x"];
+    let actual = nu!(nu_repl_code(lines));
+    assert_eq!(actual.out, "1");
+}