diffs: skip lines in diff to allow user selection from diff view.

rhodecode/public/css/code-block.less

@@ -1063,6 +1063,7 b' table.cb {'
             text-align: right;
             border-right: 1px solid #eee;
             font-family: @font-family-monospace;
+            user-select: none;
 
             a::before {
                 content: attr(data-line-no);