Don't allow the default branch to be deleted

As a delegate, you shouldn't be able to delete your default branch.
This would most likely happen by mistake, and it prevents the canonical
head from being calculated correctly.

radicle-cli/examples/git/git-push.md

@@ -61,3 +61,11 @@ f2de534b5e81d7c6e2dcaf58c3dd91573c0a0354	refs/heads/master
 $ rad sync --announce
 ✓ Synced with 1 node(s)
 ```
+
+Note that it is forbidden to delete the default/canonical branch:
+
+``` (fail) (stderr)
+$ git push rad :master
+error: refusing to delete default branch ref 'refs/heads/master'
+error: failed to push some refs to 'rad://z42hL2jL4XNk6K8oHQaSWfMgCL7ji/z6MknSLrJoTcukLrE435hVNQT4JUhbvWLX4kUzqkEStBU8Vi'
+```

radicle-remote-helper/src/push.rs

@@ -37,6 +37,9 @@ pub enum Error {
     /// Head being pushed diverges from canonical head.
     #[error("refusing to update branch to commit that is not a descendant of canonical head")]
     HeadsDiverge(git::Oid, git::Oid),
+    /// User tried to delete the canonical branch.
+    #[error("refusing to delete default branch ref '{0}'")]
+    DeleteForbidden(git::RefString),
     /// Identity document error.
     #[error("doc: {0}")]
     Doc(#[from] radicle::identity::doc::DocError),
@@ -184,6 +187,11 @@ pub fn run(
             Command::Delete(dst) => {
                 // Delete refs.
                 let refname = nid.to_namespace().join(dst);
+                let (canonical_ref, _) = &canonical;
+
+                if *dst == canonical_ref.to_ref_string() && delegates.contains(&Did::from(nid)) {
+                    return Err(Error::DeleteForbidden(dst.clone()));
+                }
                 stored
                     .raw()
                     .find_reference(&refname)