templater: use .map_or() to silence clippy without sacrificing readability

cli/src/commit_templater.rs

@@ -1027,15 +1027,8 @@ impl RefNamesIndex {
         }
     }
 
-    #[allow(unknown_lints)] // XXX FIXME (aseipp): nightly bogons; re-test this occasionally
-    #[allow(clippy::manual_unwrap_or_default)]
-    #[allow(clippy::manual_unwrap_or)] // https://github.com/rust-lang/rust-clippy/issues/13018
     pub fn get(&self, id: &CommitId) -> &[Rc<RefName>] {
-        if let Some(names) = self.index.get(id) {
-            names
-        } else {
-            &[]
-        }
+        self.index.get(id).map_or(&[], |names: &Vec<_>| names)
     }
 }