test(e2e): make gitgraph snapshots consistent

Add a commit id to 'should render a simple gitgraph with a title',
as otherwise the gitgraph renderer picks a random commit ID, and so
image snapshots will be different.

cypress/integration/rendering/gitGraph.spec.js

@@ -328,7 +328,7 @@ describe('Git Graph diagram', () => {
 title: simple gitGraph
 ---
 gitGraph
-  commit
+  commit id: "1-abcdefg"
 `,
       {}
     );