Avoid un-necessary useMutation re-renders

apollographql/react-apollo#3417 adjusted `useMutation` to make sure
the current `ApolloClient` instance is available in the result
returned when `useMutation` is first called. Unfortunately, the changes
in that PR are unintentionally modifying the `useState` based `result`
object directly, instead of using `setResult`. This is ultimately
leading to inconsistencies around the `client` instance sometimes
being included in the result and stored as the `previousResult`, and
other times not being included. This can then lead to infinite
loop / too many render problems caused by the
`!equal(this.previousResult, result)` check in `updateResult` always
passing, and the state continuously being updated by repeated calls
to `setResult`.

This commit adjusts the returned `useMutation` result to be a copy
of the original `useState` based `result`, which then includes the
`client` instance. This ensures the `useState` controlled `result`
object is not mutated outside of calling `setResult`, and avoids
the infinite loop / too many render issue.

src/react/data/MutationData.ts

@@ -42,8 +42,10 @@ export class MutationData<
   public execute(result: MutationResult<TData>) {
     this.isMounted = true;
     this.verifyDocumentType(this.getOptions().mutation, DocumentType.Mutation);
-    result.client = this.refreshClient().client;
-    return [this.runMutation, result] as MutationTuple<TData, TVariables>;
+    return [
+      this.runMutation,
+      { ...result, client: this.refreshClient().client }
+    ] as MutationTuple<TData, TVariables>;
   }
 
   public afterExecute() {