Merge Request: Changes tab can have buttons overflow from the diff header container

Summary

When the filename is big enough, it will make the buttons container shift a little bit as if it was being moved to the next line. The problem is that the overall container has a fixed size, so that creates an overflow as can be seen on the screenshot below.s

The error can be seen here: https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/13025/diffs#diff-content-a277f9443349d65d9a8ee3523b2b5cb2ad8a6567

Steps to reproduce

1. Create a filename big-enough that will shift the MR buttons to the "next line".
2. Visit the MR changes tab

Relevant logs and/or screenshots