merging unrelated branches

[David Timothy Strauss]

----- "Tom Widmer" <tom.widmer at googlemail.com> wrote:

> (you have to specify the revision range manually since there is no 
> common ancestor)

Once this happens, do they then have a common ancestor, or do you have to keep merging by manually specifying the revision range? If they don't gain common ancestor status, is there a way to create common ancestry?