Code review comment for lp:~abentley/bzr/unbreak-merge

Hi all,

This branch fixes merge so that it works when the this_tree is not a
working tree. The ConfigurableFileMerger was using merger.this_tree.branch, to
retrieve the configuration, but it should actually use merger.this_branch.

The test simply shows that generating a preview transform with a RevisionTree
as the this_tree does not raise an exception.