SQL cell: Changes to selected database don't appear in diff

[mootari]

Describe the bug

Changing the database for an SQL cell creates a new version. However, when comparing that version to the previous version, the change is not highlighted.

Additional context

Changing the database is a quite fundamental change to the behavior of a cell. This is especially dangerous when comparing a revision to the published version, as the user won't be able to notice any changes and may just republish.

For a more concrete example, this happened to me when I reviewed a notebook for which I had edit access (without realizing it at first). Once I saw the edit entry in my timeline, I tried to figure out what I had actually changed. Luckily I just reverted to the published version.