In prose diff dialogs (like "Show Details" in transactions), show "old", "new" and "diff" tabs

Summary:
Ref T7643. When you do something like this:

• Edit a task description.
• Click "Show Details" on the resulting transaction.
• Get a prose diff dialog showing the change.

...now add some "Old" and "New" tabs. These are useful for:

• reverting to the old text by copy/pasting;
• reading just the new/old text if the diff is noisy;
• sometimes just nice to have?

(This looks a little rough but I didn't want to put a negative margin on tab groups inside dialogs? Not sure what the best fix here is.)

Test Plan:

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T7643

Differential Revision: https://secure.phabricator.com/D16817