elm-pages build --base sub does not render correctly

[j-maas]

The dev server strips the base properly. But when I try to build, the index is rendered into a subfolder and contains a rendered error message about it being an invalid route. See this test HTML output for an index file for an example.

Reproduction

1. Check out https://github.com/Y0hy0h/unnecessarily-bad-design/tree/e93e004dcd3cfdc81cf67a1eb23c01d1a095ea38. (I currently don't have a simpler elm-pages repo. The ./examples don't build for me.)
2. npm install
3. npm build -- --base sub
4. Look into ./dist. There is a ./dist/sub/index.html that says "Page Not Found".

[j-maas]

I discovered that this is because Route.toPath converts the Pages.Index to /sub/, but that is put into Path.fromString which chops the trailing slash, leaving /sub.

I assume there is some code somewhere that removes the base path from these paths, but since the base path is /sub/ with a trailing slash, it does not affect /sub which is left to be wrong.