SideBySide2: Bump z-index on the "underline" cursor

Restore the z-index to 2 to make sure the cursor is on top of
intraline edits, which are colored using markText().

Change-Id: I1529768d163bd5ba5d10e5981aca619910f79d35

gerrit-gwtui/src/main/java/com/google/gerrit/client/diff/DiffTable.ui.xml

@@ -97,6 +97,7 @@ limitations under the License.
     .difftable .CodeMirror.cm-keymap-fat-cursor div.CodeMirror-cursor {
       background: transparent;
       text-decoration: underline;
+      z-index: 2;
     }
     .showtabs .cm-tab:before {
       content: "\00bb";