Duplicate previous and next commit buttons in the changes tab in merge request to bottom of page
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### Problem to solve
Next and previous commit buttons are placed at the top of page in MR changes view. It is easy to quickly move between commits, but when doing a review of MR and looking at each commit in a row - after reviewing a commit you end up at the bottom of page, and you need to go all the way up to top of page to press next commit button.
Especially if commit has big diff, or many files - scrolling up after each commit can become annoying.

### Intended users
Developers, Reviewers.
### Proposal
When "Show one file at a time" option is disabled - duplicate "previous commit" and "next commit" buttons at the bottom of page, the same way as next and previous file buttons are placed when "Show one file at a time" option is enabled.

~"type::feature"
issue