Prevent possible NPE in AuthorRevision.equals

Add check for the given object being an instance of the
expected class.

This clears a warning from Findbugs.

Change-Id: I097f8894239f04804a0038b504eccd8e2374b07a

gerrit-gwtui/src/main/java/com/google/gerrit/client/change/History.java

@@ -210,6 +210,9 @@ class History extends FlowPanel {
 
     @Override
     public boolean equals(Object o) {
+      if (!(o instanceof AuthorRevision)) {
+        return false;
+      }
       AuthorRevision b = (AuthorRevision) o;
       return author == b.author && revision == b.revision;
     }