radicle: refactor Canonical

Changes for the `Canonical` type:
- Reorder arguments so that `delegates` and `threshold` are grouped
  after `refname`
- Add `Canonical::is_empty` to detect when `Canonical` was constructed
  with no tips
- Consume `Canonical` when calling `Canonical::quorum` to indicate
  that it should be the final step

crates/radicle/src/git/canonical.rs

@@ -99,8 +99,8 @@ impl Canonical {
     {
         Self::reference(
             repo,
-            delegates,
             &lit::refs_heads(project.default_branch()).into(),
+            delegates,
             threshold,
         )
     }
@@ -109,8 +109,8 @@ impl Canonical {
     /// the reference `name`.
     pub fn reference<S>(
         repo: &S,
+        refname: &Qualified,
         delegates: &NonEmpty<Did>,
-        name: &Qualified,
         threshold: usize,
     ) -> Result<Self, raw::Error>
     where
@@ -118,14 +118,14 @@ impl Canonical {
     {
         let mut tips = BTreeMap::new();
         for delegate in delegates.iter() {
-            match repo.reference_oid(delegate, name) {
+            match repo.reference_oid(delegate, refname) {
                 Ok(tip) => {
                     tips.insert(*delegate, tip);
                 }
                 Err(e) if super::ext::is_not_found_err(&e) => {
                     log::warn!(
                         target: "radicle",
-                        "Missing `refs/namespaces/{}/{name}` while calculating the canonical reference",
+                        "Missing `refs/namespaces/{}/{refname}` while calculating the canonical reference",
                         delegate.as_key()
                     );
                 }
@@ -139,6 +139,14 @@ impl Canonical {
     pub fn tips(&self) -> impl Iterator<Item = (&Did, &Oid)> {
         self.tips.iter()
     }
+
+    /// Returns `true` is there were no tips found for any of the delegates for
+    /// the given reference.
+    ///
+    /// N.b. this may be the case when a new reference is being created.
+    pub fn is_empty(&self) -> bool {
+        self.tips.is_empty()
+    }
 }
 
 /// Check that a given `target` converges with any of the provided `tips`.
@@ -176,7 +184,7 @@ impl Canonical {
     ///
     /// Also returns an error if `heads` is empty or `threshold` cannot be
     /// satisified with the number of heads given.
-    pub fn quorum(&self, repo: &raw::Repository) -> Result<Oid, QuorumError> {
+    pub fn quorum(self, repo: &raw::Repository) -> Result<Oid, QuorumError> {
         let mut candidates = BTreeMap::<_, usize>::new();
 
         // Build a list of candidate commits and count how many "votes" each of them has.