Enable MergeableSelector in scss-lint (for !14055)

app/assets/stylesheets/framework/lists.scss

@@ -299,7 +299,8 @@ ul.indent-list {
   }
 }
 
-.group-list-tree .avatar-container.content-loading {
+.group-list-tree {
+  .avatar-container.content-loading {
     position: relative;
 
     > a,
@@ -310,15 +311,15 @@ ul.indent-list {
 
     > a {
       padding: 2px;
-  }
 
-  > a .avatar {
+      .avatar {
         border: 2px solid $white-normal;
 
         &.identicon {
           line-height: 30px;
         }
       }
+    }
 
     &::after {
       content: "";
@@ -330,9 +331,8 @@ ul.indent-list {
       border-radius: 50%;
       animation: spin-avatar 3s infinite linear;
     }
-}
+  }
 
-.group-list-tree {
   .folder-toggle-wrap {
     float: left;
     line-height: $list-text-height;