Fix Error 500 in creating merge requests with > 1000 diffs

Closes #2692

CHANGELOG

@@ -3,6 +3,7 @@ Please view this file on the master branch, on stable branches it's out of date.
 v 8.0.3
   - Fix URL shown in Slack notifications
   - Fix bug where projects would appear to be stuck in the forked import state (Stan Hu)
+  - Fix Error 500 in creating merge requests with > 1000 diffs (Stan Hu)
 
 v 8.0.2
   - Fix default avatar not rendering in network graph (Stan Hu)

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