I'm excited to share that in Bitbucket Server 7.0 we shipped redesigned and optimized pull request page with several improvements you’ve been asking us, including word-wrapping in diffs which prevents horizontal scroll bar from showing up.
You can find more information in the Bitbucket Server 7.0 release notes and Pull Requests documentation.
Please, don't hesitate to leave feedback on the feature in the comments. This will help us to priorities further development.
Product Manager Bitbucket Server