radicle: Avoid comparison with `ZERO_SHA1`

When the underlying hash function is not important, but only the fact
whether the OID is the zero sentinel, use `Oid::is_zero`. This is more
flexible, as it is forward compatible with the SHA-256 object format.

crates/radicle-oid/src/lib.rs

@@ -87,6 +87,11 @@ pub enum Oid {
 // for forwards compatibility: What if another hash with digests of the same
 // length becomes popular?
 impl Oid {
+    /// A SHA-1 object identifier with all digest bytes set to zero.
+    /// This is sometimes used as a sentinel value to indicate the absence of
+    /// an object.
+    /// To compare whether an object identifier is zero, prefer the method
+    /// [`Oid::is_zero`] over checking equality with this constant.
     pub const ZERO_SHA1: Self = Self::Sha1([0u8; SHA1_DIGEST_LEN]);
 
     pub fn from_sha1(digest: [u8; SHA1_DIGEST_LEN]) -> Self {

crates/radicle/src/storage/refs.rs

@@ -177,7 +177,7 @@ impl Refs {
         let mut buf = String::new();
 
         for (name, oid) in self.0.iter() {
-            debug_assert_ne!(oid, &Oid::ZERO_SHA1);
+            debug_assert!(!oid.is_zero());
             debug_assert_ne!(name, &SIGREFS_BRANCH.to_ref_string());
 
             buf.push_str(&oid.to_string());