Passing an argument to force an association to reload is now deprecated

What does this MR do?

Fixes deprecation: DEPRECATION WARNING: Passing an argument to force an association to reload is now deprecated and will be removed in Rails 5.1. Please call `reload_merge_request_diff` instead. (called from merge_request_diff at /builds/gitlab-org/gitlab-ce/app/models/merge_request.rb:54)

Similar to !23337 (merged)

What are the relevant issue numbers?

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• Security reports checked/validated by reviewer