cob: add method for loading entry manifest

Adds a helper for only loading the manifest of a COB entry. This allows callers
to inspect the manifest without loading the whole entry.

crates/radicle-cob/src/backend/git/change.rs

@@ -159,6 +159,12 @@ impl change::Storage for git2::Repository {
             .collect::<Vec<_>>())
     }
 
+    fn manifest_of(&self, id: &Oid) -> Result<crate::Manifest, Self::LoadError> {
+        let commit = self.find_commit(**id)?;
+        let tree = commit.tree()?;
+        load_manifest(self, &tree)
+    }
+
     fn load(&self, id: Self::ObjectId) -> Result<Entry, Self::LoadError> {
         let commit = Commit::read(self, id.into())?;
         let timestamp = git2::Time::from(commit.committer().time).seconds() as u64;

crates/radicle-cob/src/change/store.rs

@@ -38,6 +38,9 @@ pub trait Storage {
 
     /// Returns the parents of the object with the specified ID.
     fn parents_of(&self, id: &Oid) -> Result<Vec<Oid>, Self::LoadError>;
+
+    /// Load only the manifest of the change entry.
+    fn manifest_of(&self, id: &Oid) -> Result<Manifest, Self::LoadError>;
 }
 
 /// Change template, used to create a new change.

crates/radicle-cob/src/test/storage.rs

@@ -101,6 +101,10 @@ impl change::Storage for Storage {
             .map(git_ext::Oid::from)
             .collect::<Vec<_>>())
     }
+
+    fn manifest_of(&self, id: &git_ext::Oid) -> Result<crate::Manifest, Self::LoadError> {
+        self.as_raw().manifest_of(id)
+    }
 }
 
 impl object::Storage for Storage {

crates/radicle/src/storage/git/cob.rs

@@ -82,6 +82,10 @@ impl change::Storage for Repository {
     fn parents_of(&self, id: &Oid) -> Result<Vec<Oid>, Self::LoadError> {
         self.backend.parents_of(id)
     }
+
+    fn manifest_of(&self, id: &Oid) -> Result<cob::Manifest, Self::LoadError> {
+        self.backend.manifest_of(id)
+    }
 }
 
 impl cob::object::Storage for Repository {
@@ -223,6 +227,10 @@ impl<R: storage::WriteRepository> change::Storage for DraftStore<'_, R> {
     fn parents_of(&self, id: &Oid) -> Result<Vec<Oid>, Self::LoadError> {
         self.repo.raw().parents_of(id)
     }
+
+    fn manifest_of(&self, id: &Oid) -> Result<cob::Manifest, Self::LoadError> {
+        self.repo.raw().manifest_of(id)
+    }
 }
 
 impl<R> SignRepository for DraftStore<'_, R>

crates/radicle/src/test/storage.rs

@@ -440,4 +440,8 @@ impl radicle_cob::change::Storage for MockRepository {
     fn parents_of(&self, _id: &Oid) -> Result<Vec<Oid>, Self::LoadError> {
         todo!()
     }
+
+    fn manifest_of(&self, _id: &Oid) -> Result<radicle_cob::Manifest, Self::LoadError> {
+        todo!()
+    }
 }