"View Diff" on online editor shows all lines are removed for Mercurial repositories (BBCDEV-7363)

Steps to reproduce:

1. Create a Mercurial repository
2. Create a README.md
3. Commit
4. Edit README.md using the online editor
5. Change 1 character
6. Click on View Diff in the editor

The diff would show that all lines are removed. However, this does not reflect the actual commit done and is purely a UI issue.