Fix NPE in unified diff view when creating a comment editor

We failed to initialize the versions table, so comment editor
creation in the unified diff view of a file always failed with
a null pointer exception.  In a unified diff view we currently
assume there are only two file versions (base, patch set).

Signed-off-by: Shawn O. Pearce <sop@google.com>

src/main/java/com/google/gerrit/client/patches/UnifiedDiffTable.java

@@ -77,6 +77,8 @@ public class UnifiedDiffTable extends AbstractPatchContentTable {
   }
 
   public void display(final List<PatchLine> list) {
+    initVersions(2);
+
     final StringBuilder nc = new StringBuilder();
     for (final PatchLine pLine : list) {
       appendLine(nc, pLine);