Fix alternate search paths.

Alternate search paths are currently not working correctly. With multiple blocks, the same search path is used for both. For example, in the following `index.html`

    <!-- build:js scripts/vendor.js -->
      <script src="vendor/jquery/dist/jquery.js"></script>
      <script src="vendor/bootstrap-sass/dist/js/bootstrap.js"></script>
    <!-- endbuild -->

    <!-- build:js(.tmp) scripts/app.js -->
      <script src="index.js"></script>
    <!-- endbuild -->

The search path should be `.tmp` only for the second block, but it also affects the first block. You will get an error on `gulp-useref/index.js:54:44`, because `filepath` on line 52 is `/path/to/app/.tmp`, but it's actually looking for `vendor/jquery/dist/jquery.js`.

This patch fixes the problem, adds a passing test and cleans up the code branching that was added before that was not needed.