…are about

That was a pain to debug as axum only traces it in... well debug[^1],
otherwise you just get a 500 with no information.

I'd always assumed that

    Router::new()
        .nest("/{foo}/{bar}",
            Router:new()
                .route(...)
                .route(...)
                .layer(map_request(map_fn)))

would receive just the `Path` from the parent router, but apparently
it "inherits" the path from the route being matched, meaning the
`Path((_, _))` of `normalize_repo_name` would receive *4* components
if the request was to `/objects`, but would ignore the last two.

tokio-rs/axum#2931 changed this behaviour such that `Path<Tuple>` now
only allows exact extractions, and this applies to `map_request`'s
handler.

In this case we specifically know that we only care about the `name`
of the repository, so we can leverage a bespoke `struct` and that
serde ignores extra / unknown fields by default, without needing a
more expensive deserialization target (e.g. a full hashmap).

[^1]: and not precisely, axum does mention the `Path` size mismatch
    but doesn't say where it happens.