fix firefox zclip bug

templates/repo/single_bare.tmpl

@@ -16,7 +16,7 @@
                         <button class="btn btn-default" data-link="{{.CloneLink.HTTPS}}" type="button">HTTPS</button>
                     </span>
                     <input type="text" class="form-control clone-group-url" id="guide-clone-url" value="" readonly/>
-                    <span class="input-group-btn">
+                    <span class="input-group-btn" style="position: relative">
                         <button class="btn btn-default" type="button" data-toggle="tooltip" title="copy to clipboard" data-placement="top" data-init="copy" data-copy-val="val" data-copy-from="#guide-clone-url"><i class="fa fa-copy"></i></button>
                     </span>
                 </div>