Unbreak fingerprinting when called twice for the same transaction set

Unbreak fingerprinting when called twice for the same transaction set

- Fixes regression (yum updates barf with conflicts) from commit
  3b492620fb35a21d05b975e31130dc071f6fd8e2 which doesn't affect rpm
  itself but breaks some API users like yum that first run a
  test-transaction and if that succeeds, the same transaction is run
  for real. In that case we need to redo the whole fingerprint dance
  from scratch: throw away any former results and redo.
- Blaming Fedora 18 alpha preparations for not noticing this earlier:
  there have been no updates to F16/F17 in almost a week, effectively
  disabling a part of the regular rpm development QA :P