Resolve "Expanding a diff while having an open comment form will always scroll down to the comment"

What does this MR do?

Diff lines were previously using the index of a line, which caused a lot of unnecessary remounting instead of allowing Vue to properly only rerender things that had changed. Using the LineCode stops the mounted method from being called unnecessarily, and therefore stops the textarea from being focused when loading more lines.

What are the relevant issue numbers?

Closes #51606 (closed)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Tests added for this feature/bug
• Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the style guides
• Security reports checked/validated by reviewer