Add ",codemirror" view of a file

To support development of an alternative view of files using
CodeMirror any patch can now be viewed using CodeMirror by
replacing ",unified" in the URL with ",codemirror".

This is currently fairly boring. I did just enough glue to
initialize the CodeMirror object, supply it with file data,
attach it to the DOM, and let it render. There are no modes
loaded so there is currently no syntax highlighting or any
other fancy features.

The intent of this new screen is to allow development in-tree.
If this ever becomes fully functional it will replace current
views by updating the Dispatcher. If it fails we will just rip
the code out and try something else.

Change-Id: I0a187b4ff2e273b803500af97d2bb1772f166daf

gerrit-gwtui/src/main/java/com/google/gerrit/client/diff/DiffApi.java

@@ -37,7 +37,9 @@ public class DiffApi {
   }
 
   public DiffApi base(PatchSet.Id id) {
-    call.addParameter("base", id.get());
+    if (id != null) {
+      call.addParameter("base", id.get());
+    }
     return this;
   }