httpd: accept `/{rid}.git/{nid}/` paths as `GIT_NAMESPACE`-d remotes — radicle-explorer

This patch adds support for serving /{rid}/{nid}/... paths as GIT_NAMESPACE-d http remotes, using {nid} as the namespace. Radicle’s on-disk repo storage already uses a Git-namespace-compatible notion for peer refs (refs/namespaces/*), so it is merely a matter of passing the right $GIT_NAMESPACE to the git http-backend process.

This way, it becomes possible to directly clone or fetch from URLs like http://httpd.domain.tld/z3gqcJUoA1n9HaHKufZs5FCSGazv5.git/z6MkireRatUThvd3qzfKht1S44wpm4FEWSSa4PRMTSQZ3voM to directly access the given node’s ref namespace:

$ cd radicle-explorer
$ git remote add intelfx https://iris.radicle.xyz/z4V1sjrXqjvFdnCUbxPFqd5p4DtH5/z6MkqbVudjUcuxUmwWfW6U886CnhoX5wmkEDsAXju9F9UBAo
$ git pull intelfx work/httpd-git-namespace // or fetch, or ...

Instead of mucking about with

$ git fetch origin refs/namespaces/{nid}/refs/heads/{branch}:refs/heads/{branch}

 use hyper::body::Buf as _;
 use radicle::identity::RepoId;
 use radicle::node::NodeId;
 use radicle::profile::Profile;
 use radicle::storage::{ReadRepository, ReadStorage};

         }
     };
     let (nid, request): (Option<NodeId>, &str) = {
         let (first, rest) = request.split_once('/').unwrap_or((&request, ""));
         match first.parse::<NodeId>() {
             Ok(nid) => (Some(nid), rest),
             Err(_) => (None, &request),
         }
     };
     let (status, headers, body) = git_http_backend(
         &profile, method, headers, body, remote, rid, &request, query,
         &profile, method, headers, body, remote, rid, nid, &request, query,
     )
     .await?;

     mut body: Bytes,
     remote: DualAddr,
     id: RepoId,
     nid: Option<NodeId>,
     path: &str,
     query: String,
 ) -> Result<(StatusCode, HashMap<String, Vec<String>>, Vec<u8>), Error> {

     tracing::debug!("remote: {:?}", remote);
     let mut cmd = Command::new("git");
     if let Some(nid) = nid {
         cmd.env("GIT_NAMESPACE", nid.to_string());
     }
     let mut child = cmd
         // This is a workaround to allow fetching particular commits by their OID.
         // Otherwise, the client errors with "Server does not allow request for unadvertised object"