merge-upstream blows up if a criss cross is found

[Robert Collins]

This causes a 'no merge base' error because there are two lcas found.

Its a little complex, but I have

debian:
debtrunk
 upstream-0.1
maintrunk
...

upstream;
newstuff
maintrunk
...

So the import does:

upstream-0.2: [upstream-0.1, newstuff]
and then a merge of upstream-0.2 into debian: find_lca finds
upstream-0.1 and maintrunk as common ancestors. However, because
upstream-0.1 does not have maintrunk in its ancestry an error is thrown
- no merge base.

This is arguably an error in my tree - I blogged
http://rbtcollins.wordpress.com/2009/12/19/debianising-with-bzr-builddeb/ about debianising with bzr-builddeb, and it appears that the upstream import it creates is missing a link to the trunk. (This is another reason to have a few smaller steps exposed so the hackery in that blog post is not needed and we can get cleaner data).

However, I think it might be nice to, when a no-merge-base is found
during merge-upstream, force the previous upstream tarball as a common
base, because we should expect it to be there and sane.

I'm looking at whether this is easy or not now.

 affects bzr-builddeb