Merge Request doesn't show overflow message

Description

We don't show an overflow message when creating a merge request where the 'expand all' version of the compare view would show the overflow message:

Saving the MR diffs is exactly equivalent to doing 'expand all' on the compare view. That's because when we expand MR diffs, we look them up in the serialised object in the database. If we don't show a message in this case, it's very confusing to users.

Original description

Summary

When I create new Merge Request, at New Merge Request screen it shows

Showing 310 changed files with 4571 additions and 297 deletions

But at Merge Request details screen, it becomes

Showing 167 changed files with 4922 additions and 7 deletions

Steps to reproduce

Create Merge Request to reproduce

Expected behavior

Merge Request shows enough changed files

Actual behavior

Merge Request doesn't show enough changed files

Relevant logs and/or screenshots

New Merge Request screen --> http://prntscr.com/dcmqmp Merge Request details --> http://prntscr.com/dcmq1r

Output of checks

Results of GitLab application Check

(For installations with omnibus-gitlab package run and paste the output of: sudo gitlab-rake gitlab:check SANITIZE=true)

(For installations from source run and paste the output of: sudo -u git -H bundle exec rake gitlab:check RAILS_ENV=production SANITIZE=true)

(we will only investigate if the tests are passing)

Results of GitLab environment info

(For installations with omnibus-gitlab package run and paste the output of: sudo gitlab-rake gitlab:env:info)

(For installations from source run and paste the output of: sudo -u git -H bundle exec rake gitlab:env:info RAILS_ENV=production)

Possible fixes

(If you can, link to the line of code that might be responsible for the problem)