Resolve "Long file names in MR diffs cause horizontal scrolling"

What does this MR do?

Applying mx-auto to diff files was causing problems on small devices. It forced the diff file to expand and sometimes caused horizontal scrolling - other times it caused the diff file to overlap with the right sidebar.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Style guides

Performance and testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers

Closes #60304 (closed)