node: handle removal of cob from cache — heartwood

When a Patch or Issue is entirely removed from the Git repository, then there is no update to be made – instead it should also be removed from the cache.

This covers the case where the original author of the COB deletes the COB.

 ``` ~alice (stderr)
 $ git push rad -o patch.draft -o patch.message="Define LICENSE for project" HEAD:refs/patches
 ✓ Patch 6c61ef1716ad8a5c11e04dd7a3fec51e01fba70b drafted
 ✓ Synced with 1 node(s)
 ✓ Synced with 2 node(s)
 To rad://z42hL2jL4XNk6K8oHQaSWfMgCL7ji/z6MknSLrJoTcukLrE435hVNQT4JUhbvWLX4kUzqkEStBU8Vi
  * [new reference]   HEAD -> refs/patches
 ```

 │ bob (you) now 833db19     │
 │ I think we should use MIT │
 ╰───────────────────────────╯
 ✓ Synced with 1 node(s)
 ✓ Synced with 2 node(s)
 ```
 ``` ~alice

 │ alice (you) now 1803a38 │
 │ Thanks, I'll add it!    │
 ╰─────────────────────────╯
 ✓ Synced with 1 node(s)
 ✓ Synced with 2 node(s)
 ```
 ``` ~alice

 ``` ~alice (stderr)
 $ git push -f
 ✓ Patch 6c61ef1 updated to revision 93915b9afa94a9dc4f52f12cdf077d4613ea3eb3
 ✓ Synced with 1 node(s)
 ✓ Synced with 2 node(s)
 To rad://z42hL2jL4XNk6K8oHQaSWfMgCL7ji/z6MknSLrJoTcukLrE435hVNQT4JUhbvWLX4kUzqkEStBU8Vi
 c900..1cc8cd9  prepare-license -> patches/6c61ef1716ad8a5c11e04dd7a3fec51e01fba70b
 ```

 ``` ~bob
 $ rad patch review 6c61ef1 --accept -m "LGTM!"
 ✓ Patch 6c61ef1 accepted
 ✓ Synced with 1 node(s)
 ✓ Synced with 2 node(s)
 $ rad patch show 6c61ef1 -v
 ╭─────────────────────────────────────────────────────────────────────╮
 │ Title    Define LICENSE for project                                 │

 │ ↑ updated to 93915b9afa94a9dc4f52f12cdf077d4613ea3eb3 (1cc8cd9) now │
 ╰─────────────────────────────────────────────────────────────────────╯
 ```
 If Alice also decides to delete the patch, then any seeds that have synced with
 Alice should no longer have the patch:
 ``` ~alice
 $ rad patch delete 6c61ef1
 ✓ Synced with 2 node(s)
 ```
 ``` ~seed (fails)
 $ rad patch show --repo rad:z42hL2jL4XNk6K8oHQaSWfMgCL7ji 6c61ef1 -v
 ✗ Error: Patch `6c61ef1716ad8a5c11e04dd7a3fec51e01fba70b` not found
 ```

     let mut environment = Environment::new();
     let alice = environment.node(Config::test(Alias::new("alice")));
     let bob = environment.node(Config::test(Alias::new("bob")));
     let seed = environment.node(Config::test(Alias::new("seed")));
     let working = environment.tmp().join("working");
     let acme = RepoId::from_str("z42hL2jL4XNk6K8oHQaSWfMgCL7ji").unwrap();

     let mut alice = alice.spawn();
     let mut bob = bob.spawn();
     let mut seed = seed.spawn();
     bob.handle.seed(acme, Scope::All).unwrap();
     alice.connect(&bob).converge([&bob]);
     seed.handle.seed(acme, Scope::All).unwrap();
     alice.connect(&bob).connect(&seed).converge([&bob, &seed]);
     test(
         "examples/rad-clone.md",

             working.join("bob"),
             [("RAD_HOME", bob.home.path().display())],
         )
         .home(
             "seed",
             working.join("seed"),
             [("RAD_HOME", seed.home.path().display())],
         )
         .run()
         .unwrap();
 }

                                     type_name: identifier.type_name,
                                     err: e.into(),
                                 })?;
                         } else {
                             // N.b. the issue has been removed entirely from the
                             // repository so we also remove it from the cache
                             cob::cache::Remove::<cob::issue::Issue>::remove(cache, &identifier.id)
                                 .map(|_| ())
                                 .map_err(|e| error::Cache::Remove {
                                     id: identifier.id,
                                     type_name: identifier.type_name,
                                     err: Box::new(e),
                                 })?;
                         }
                     } else if identifier.is_patch() {
                         if let Some(patch) = patches.get(&identifier.id)? {