Fix merge request comments on files with multiple commits

Having a merge request with a comments on a line which is then changed
in a later commit prevented new comments from properly showing up in the
merge request show page.

 * `Note#set_diff` do not use stored the diff when creating a new note
   in merge requests (we can not be sure the diff did not changed since
   the last comment on that line)
 * Do not rely just on `outdated?` of the first note when displaying
   comments of a MR in the discussion tab, but partition the
   active/outdated notes and display them all
 * In the inline changes tab just select the active notes, so an
   outdated note does not prevent an active one from being rendered
 * Just show active comments in the side-by-side changes tab

1 files changed, 5 insertions, 1 deletions

diff --git a/app/models/note.rb b/app/models/note.rb
index fdab4517df3..2cf3fac2def 100644
--- a/app/models/note.rb
+++ b/app/models/note.rb
@@ -354,7 +354,7 @@ class Note < ActiveRecord::Base
   def set_diff
     # First lets find notes with same diff
     # before iterating over all mr diffs
-    diff = Note.where(noteable_id: self.noteable_id, noteable_type: self.noteable_type, line_code: self.line_code).last.try(:diff)
+    diff = diff_for_line_code unless for_merge_request?
     diff ||= find_diff
 
     self.st_diff = diff.to_hash if diff
@@ -364,6 +364,10 @@ class Note < ActiveRecord::Base
     @diff ||= Gitlab::Git::Diff.new(st_diff) if st_diff.respond_to?(:map)
   end
 
+  def diff_for_line_code
+    Note.where(noteable_id: noteable_id, noteable_type: noteable_type, line_code: line_code).last.try(:diff)
+  end
+
   # Check if such line of code exists in merge request diff
   # If exists - its active discussion
   # If not - its outdated diff