Always use single-column diffs if there are no deletions *or* no additions #4397

<img width="925" alt="Screen Shot 2" src="https://user-images.githubusercontent.com/1402241/119452318-65f50a80-bd60-11eb-9f83-b49e47ea21f5.png">

Sometimes split diffs are useless because the diffs appear on only one column, like in the files of these PRs:

• https://github.com/sindresorhus/refined-github/pull/4296/files?diff=split (every file)
• https://github.com/sindresorhus/refined-github/pull/3637/files?diff=split (every file)
• https://github.com/sindresorhus/refined-github/pull/4382/files?diff=split (only 2 files of 5)

Showing the diff on 2 columns just limits the available space for code so I think that, if easily possible, we should hide one column in the files that don't benefit from split views.