Factor .add-diff-note active state.

app/assets/stylesheets/sections/notes.scss

@@ -143,6 +143,11 @@ ul.notes {
  */
 
 .diff-file tr.line_holder {
+  @mixin show-add-diff-note {
+    filter: alpha(opacity=100);
+    opacity: 1.0;
+  }
+
   .add-diff-note {
     background: image-url("diff_note_add.png") no-repeat left 0;
     height: 22px;
@@ -156,8 +161,7 @@ ul.notes {
     filter: alpha(opacity=0);
 
     &:hover {
-      opacity: 1.0;
-      filter: alpha(opacity=100);
+      @include show-add-diff-note;
     }
   }
 
@@ -166,8 +170,7 @@ ul.notes {
     background: $hover !important;
 
     .add-diff-note {
-      opacity: 1.0;
-      filter: alpha(opacity=100);
+      @include show-add-diff-note;
     }
   }
 }