[MERGE] Refactor fetch: add get_data_about_revision_ids to repository, and other changes.

[Andrew Bennetts]

Aaron Bentley wrote:
> Aaron Bentley has voted comment.
> Status is now: Conditionally approved
> Comment:
> Please use revision.is_null in new code, rather than in (None, 
> NULL_REVISION)

Well, it's old code that was reorganised slightly rather than new code, but I've
made this change.  The new condition is now:

        if last_revision is not None and not is_null(last_revision):

Because last_revision may be None, because in this API that means "fetch
everything".

Thanks for the review!

-Andrew.