Add space to separate different files in diff

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

app/assets/stylesheets/pages/diff.scss

 // Common
 .diff-file {
   border: 1px solid $border-color;
-  border-top: none;
+  margin-bottom: $gl-padding;
 
   .diff-header {
     position: relative;
@@ -361,3 +361,11 @@
     border-color: $border;
   }
 }
+
+.files {
+  margin-top: -1px;
+
+  .diff-file:last-child {
+    margin-bottom: 0;
+  }
+}