build failure

[Joern Rennecke]

> (not being able to rename files without losing a sense of history comes to 
> mind.  Not being able to use import on anything but the trunk as well)

It would also make sense to be able to re-base a branch on a new branchpoint.
That would prevent degradation where a branch continues mostly, but not quite,
merged into the future, and you want to access the newest versions in general.