Merge.js multiple `override:` entries

[zachleat]

Originally posted by @Snapstromegon #2470 (comment)

  t.deepEqual(
    Merge(
      {
        a: {
          "override:b": {
            c: [1],
          },
        },
      },
      {
        a: {
          "override:b": {
            c: [2],
          },
        },
      }
    ),
    {
      a: {
        b: {
          c: [1,2],
        },
        "override:b": {
          c: [1,2],
        },
      },
    }
  );