ref-format: Relax first component of namespaced — radicle-git

Git supports dividing the refs of a single repository into multiple namespaces, each of which has its own branches, tags, and HEAD.

Here, the mention of HEAD implies that requiring all namespaced to start with

refs/namespaces/*/refs

is too strict, as there are legal namespaced refs such as

refs/namespaces/*/HEAD

which are not captured. Relax the corresponding condition, change documentation and add a test to avoid regression.

 /// A [`Qualified`] ref under a git namespace.
 ///
 /// A ref is namespaced if it starts with "refs/namespaces/", another path
 /// component, and "refs/". Eg.
 /// component, and "refs" or "HEAD". Eg.
 ///
 ///     refs/namespaces/xyz/refs/heads/main
 ///     refs/namespaces/xyz/HEAD
 ///
 /// Note that namespaces can be nested, so the result of
 /// [`Namespaced::strip_namespace`] may be convertible to a [`Namespaced`]

     fn from(rs: &'a RefStr) -> Self {
         let mut cs = rs.iter();
         match (cs.next()?, cs.next()?, cs.next()?, cs.next()?) {
             ("refs", "namespaces", _, "refs") => Some(Namespaced(Cow::from(rs))),
             ("refs", "namespaces", _, "refs" | "HEAD") => Some(Namespaced(Cow::from(rs))),
             _ => None,
         }

 }
 #[test]
 fn namespaced_head() {
     assert_eq!(
         "refs/namespaces/foo/HEAD",
         refname!("refs/namespaces/foo/HEAD")
             .to_namespaced()
             .unwrap()
             .as_str()
     )
 }
 #[test]
 fn not_namespaced() {
     assert!(name::REFS_HEADS_MAIN.to_namespaced().is_none())
 }