remote-helper: Don't allow opening an empty patch

If the commit range is empty, return an error.

radicle-cli/examples/rad-patch-via-push.md

@@ -222,3 +222,15 @@ $ rad patch show b8ab1c9
 │ ↑ updated to f24334f8cea7b7a5bcaf3bc6deb1408c9bf507ad (9304dbc) [   ...    ] │
 ╰──────────────────────────────────────────────────────────────────────────────╯
 ```
+
+## Empty patch
+
+If we try to open a patch without making any changes to our base branch (`master`),
+we should get an error:
+
+``` (stderr) (fail)
+$ git push rad master:refs/patches
+To rad://z42hL2jL4XNk6K8oHQaSWfMgCL7ji/z6MknSLrJoTcukLrE435hVNQT4JUhbvWLX4kUzqkEStBU8Vi
+ ! [remote rejected] master -> refs/patches (patch commits are already included in the base branch)
+error: failed to push some refs to 'rad://z42hL2jL4XNk6K8oHQaSWfMgCL7ji/z6MknSLrJoTcukLrE435hVNQT4JUhbvWLX4kUzqkEStBU8Vi'
+```

radicle-remote-helper/src/push.rs

@@ -71,6 +71,9 @@ pub enum Error {
     /// Patch not found in store.
     #[error("patch `{0}` not found")]
     NotFound(patch::PatchId),
+    /// Patch is empty.
+    #[error("patch commits are already included in the base branch")]
+    EmptyPatch,
     /// COB store error.
     #[error(transparent)]
     Cob(#[from] radicle::cob::store::Error),
@@ -259,6 +262,9 @@ fn patch_open<G: Signer>(
 
     let (_, target) = stored.canonical_head()?;
     let base = stored.backend.merge_base(*target, commit.id())?;
+    if base == commit.id() {
+        return Err(Error::EmptyPatch);
+    }
     let (title, description) = cli::patch::get_create_message(
         opts.message,
         &stored.backend,