Fixed a bug with ReportMergeScenarioOp and RevTreeImpl.

Fixed a bug in ReportMergeScenarioOp where removed features weren't being added to the unconflicted list.
The previously stated bug fix caused a test to fail which revealed a problem with the RevTreeImpl class where if the tree was empty when it was read in it would be created with an empty bucket list instead of Optional.absent() which was causing it to list a feature as removed twice when DiffTree was called.