Merge branch 'fix-500-compare' into 'master'

Fix  random 500 error on compare page
Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

Bug appeared after recent mr !1471  

See merge request !1486

app/helpers/diff_helper.rb

@@ -170,7 +170,7 @@ module DiffHelper
 
   def commit_for_diff(diff)
     if diff.deleted_file
-      @merge_request ? @merge_request.commits.last : @commit.parent_id
+      @merge_request ? @merge_request.commits.last : @commit.parents.first
     else
       @commit
     end