Fix whitespace between numbers and their signs [Rapid Diffs][Diff File]

What does this MR do and why?

• In support of Rapid Diffs - a feature-flagged rewrite (rapid_diffs)
• For part of #498890 (closed)

---

• Removes source whitespace from output HTML.
• Swaps for the correct negative symbol (MINUS_SIGN versus HYPHEN-MINUS).
• Adds spacing between the added and removed numbers.

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

Before (Production)	Before (Rapid Diffs)	After (Rapid Diffs)

How to set up and validate locally

1. Enable the rapid diffs feature flag in the Rails console
   1. rails c
   2. Feature.enable( :rapid_diffs )
2. Have - or create - an MR with at least 1 changed line
3. View the MR's Changes tab
4. Note the line counts (Do not note anything else!)