Reverting a vendor-branch import

[npat]

I have performed a vendor-barnch import. I have started the merging
procedure, and half way to this I want to "revert" (undo) the whole
import thing. Before doing the import I have tagged my tree with, say,
T1. How can I do it?

/npat

-- 
Freedom, morality, and the human dignity of the individual consists
precisely in this; that he does good not because he is forced to do
so, but because he freely conceives it, wants it, and loves it.
  -- Mikhail Bakunin