radicle: add Op::manifest_of

Add a helper method `Op::manifest_of`, to allow the caller to inspect the
`Manifest` of the entry. This can be used to avoid attempting to call `Op:load`
with a manifest that would not match the expected `Action`s of the `Op`.

crates/radicle/src/cob/op.rs

@@ -3,10 +3,10 @@ use radicle_cob::Manifest;
 use serde::Serialize;
 use thiserror::Error;
 
-use radicle_cob as cob;
 use radicle_cob::history::{Entry, EntryId};
 use radicle_crypto::PublicKey;
 
+use crate::cob;
 use crate::cob::Timestamp;
 use crate::identity::DocAt;
 use crate::storage::ReadRepository;
@@ -24,6 +24,14 @@ pub enum OpEncodingError {
     Git(#[from] git2::Error),
 }
 
+#[derive(Error, Debug)]
+#[error("failed to load manifest of '{object}': {err}")]
+pub struct ManifestError {
+    object: git::Oid,
+    #[source]
+    err: Box<dyn std::error::Error + Send + Sync + 'static>,
+}
+
 /// Error loading an `Op` from storage.
 #[derive(Error, Debug)]
 pub enum LoadError {
@@ -112,6 +120,20 @@ impl<A> Op<A> {
         }
     }
 
+    pub fn manifest_of<S>(store: &S, id: &git::Oid) -> Result<Manifest, ManifestError>
+    where
+        S: cob::change::Storage<
+            ObjectId = git::Oid,
+            Parent = git::Oid,
+            Signatures = crypto::ssh::ExtendedSignature,
+        >,
+    {
+        store.manifest_of(id).map_err(|err| ManifestError {
+            object: *id,
+            err: Box::new(err),
+        })
+    }
+
     /// Get the `Op` identified by the `id` in the provided `store`.
     pub fn load<S>(store: &S, id: git::Oid) -> Result<Self, LoadError>
     where