Merge branch 'diff-white-line-fix' into 'master'

Removes white line when hiding diff comments in MR

Fixes #6071 

See merge request !2407

app/assets/stylesheets/pages/diff.scss

@@ -66,6 +66,7 @@
       width: 100%;
       font-family: $monospace_font;
       border: none;
+      border-collapse: separate;
       margin: 0px;
       padding: 0px;
       .line_holder td {