[MERGE] Branch format 6

[Nicholas Allen]

> I'm not sure 'strict history' really conveys the meaning of this though.
> In that bug john suggests 'preserve_mainline' -- how is that for you?
> Or maybe 'disallow_mainline_changes'.  I would like to convey the facts
> that 
>
>   this option disallows operations that would otherwise be allowed
>
>   and the operation is anything that changes the mainline revision 
>   history (is there a standard user-visible term for that now?)
>   
How about 'preserve_revision_numbers'? That's how it appears to the user 
that the revnos don't change.

Or maybe 'append_revisions_only' , 'append_revision_numbers' , 
'keep_revision_numbers'....

Nick