Improve sticky headers in diffs

* disable sticky headers in discussion
* enable sticky header on mr page with you click changes tab

app/assets/javascripts/diff.js.coffee

@@ -37,8 +37,6 @@ class @Diff
       )
     )
 
-    $('.diff-header').stick_in_parent(recalc_every: 1, offset_top: $('.navbar').height())
-
   lineNumbers: (line) ->
     return ([0, 0]) unless line.children().length
     lines = line.children().slice(0, 2)

app/views/projects/diffs/_diffs.html.haml

-.row.prepend-top-20.append-bottom-10
-  .col-md-8
-    = render 'projects/diffs/stats', diffs: diffs
-  .col-md-4
-    .btn-group.pull-right
+.prepend-top-20.append-bottom-20
+  .pull-right
+    .btn-group
       = inline_diff_btn
       = parallel_diff_btn
+  = render 'projects/diffs/stats', diffs: diffs
 
 - if show_diff_size_warning?(diffs)
   = render 'projects/diffs/warning', diffs: diffs
@@ -19,3 +18,6 @@
       Failed to collect changes
     %p
       Maybe diff is really big and operation failed with timeout. Try to get diff locally
+
+:coffeescript
+  $('.files .diff-header').stick_in_parent(recalc_every: 1, offset_top: $('.navbar').height())