remote-helper: Fix refs advertisement for patches

Some of the old patches had missing objects and that causes problems
during the ref advertisement, as the object is not found.

In this patch, we check that the object exists and also that the patch
is still open. Otherwise a remote tracking branch is created for every
single patch ref.

radicle-cli/examples/workflow/6-pulling-contributor.md

@@ -23,7 +23,6 @@ Your branch is up to date with 'rad/master'.
 $ git pull --all --ff
 From rad://z42hL2jL4XNk6K8oHQaSWfMgCL7ji
    f2de534..f567f69  master     -> rad/master
-   27857ec..f567f69  patches/50e29a111972f3b7d2123c5057de5bdf09bc7b1c -> rad/patches/50e29a111972f3b7d2123c5057de5bdf09bc7b1c
 From rad://z42hL2jL4XNk6K8oHQaSWfMgCL7ji/z6MknSLrJoTcukLrE435hVNQT4JUhbvWLX4kUzqkEStBU8Vi
    f2de534..f567f69  master     -> alice@z6MknSLrJoTcukLrE435hVNQT4JUhbvWLX4kUzqkEStBU8Vi/master
 ```

radicle-remote-helper/src/list.rs

@@ -72,7 +72,11 @@ fn patch_refs<R: ReadRepository + cob::Store>(stored: &R) -> Result<(), Error> {
     let patches = radicle::cob::patch::Patches::open(stored)?;
     for patch in patches.all()? {
         let (id, patch) = patch?;
+        let head = patch.head();
+
+        if patch.is_open() && stored.commit(*head).is_ok() {
             println!("{} {}", patch.head(), git::refs::storage::patch(&id));
         }
+    }
     Ok(())
 }

radicle/src/storage.rs

@@ -391,7 +391,7 @@ pub trait ReadRepository: Sized {
 
     /// Get the [`git2::Commit`] found using its `oid`.
     ///
-    /// Returns `None` if the commit did not exist.
+    /// Returns `Err` if the commit did not exist.
     fn commit(&self, oid: Oid) -> Result<git2::Commit, git::ext::Error>;
 
     /// Perform a revision walk of a commit history starting from the given head.