Resolve "On master, inline diff view contains extra `+` and `-` signs"

What does this MR do?

It fixes a problem on the Inline Diff view where we didn't remove the extra +/- anymore as trimFirstCharOfLineContent was doing everything against a clone and returning a new object now, which was not reapplied to the highlightedDiffLines.

What are the relevant issue numbers?

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides

Closes #51281 (closed)