cli: fetch if commmit missing when `rad merge`ing

Reduce friction when merging patches by automatically fetching the
Patch's commit head if its missing.

The calls git directly due to difficulties fetch the revision
anonymously with existing method.  Its important to fetch the commit
anonymously to remove dependency on how the local 'git remote's are
configured.

radicle-cli/src/commands/merge.rs

@@ -177,6 +177,24 @@ pub fn run(options: Options, ctx: impl term::Context) -> anyhow::Result<()> {
     //
     let patch_commit = repo
         .find_annotated_commit(revision.head().into())
+        .or_else(|e| match e.code() {
+            git::raw::ErrorCode::NotFound => {
+                // Avoid using git2 until 'rad://' supports fetching using an Oid as a refspec.
+                crate::git::git(
+                    repo.path(),
+                    [
+                        "fetch",
+                        &git::Url::from(id)
+                            .with_namespace(**patch.author().id())
+                            .to_string(),
+                        &revision.head().to_string(),
+                    ],
+                )?;
+                let c = repo.find_annotated_commit(revision.head().into())?;
+                Ok(c)
+            }
+            _ => Err(anyhow::Error::from(e)),
+        })
         .context("patch head not found in local repository")?;
     let (merge, _merge_pref) = repo.merge_analysis(&[&patch_commit])?;