Fix Error 500 in creating merge requests with > 1000 diffs

Closes #2692

CHANGELOG

@@ -1,6 +1,7 @@
 Please view this file on the master branch, on stable branches it's out of date.
 
 v 8.1.0 (unreleased)
+  - Fix Error 500 in creating merge requests with > 1000 diffs (Stan Hu)
   - Show CI status on all pages where commits list is rendered
   - Automatically enable CI when push .gitlab-ci.yml file to repository
   - Move CI charts to project graphs area

app/models/merge_request_diff.rb

@@ -123,12 +123,12 @@ class MergeRequestDiff < ActiveRecord::Base
     if new_diffs.any?
       if new_diffs.size > Commit::DIFF_HARD_LIMIT_FILES
         self.state = :overflow_diff_files_limit
-        new_diffs = new_diffs.first[Commit::DIFF_HARD_LIMIT_LINES]
+        new_diffs = new_diffs.first(Commit::DIFF_HARD_LIMIT_LINES)
       end
 
       if new_diffs.sum { |diff| diff.diff.lines.count } > Commit::DIFF_HARD_LIMIT_LINES
         self.state = :overflow_diff_lines_limit
-        new_diffs = new_diffs.first[Commit::DIFF_HARD_LIMIT_LINES]
+        new_diffs = new_diffs.first(Commit::DIFF_HARD_LIMIT_LINES)
       end
     end