Fix z index bugs

app/assets/stylesheets/pages/issuable.scss

@@ -193,7 +193,6 @@
   top: $header-height;
   bottom: 0;
   right: 0;
-  z-index: 8;
   transition: width .3s;
   background: $gray-light;
   padding: 10px 20px;

app/assets/stylesheets/pages/tree.scss

@@ -171,6 +171,8 @@
 .tree-controls {
   float: right;
   margin-top: 11px;
+  position: relative;
+  z-index: 2;
 
   .project-action-button {
     margin-left: $btn-side-margin;