Workaround for dir-changed-to-file updates mentioned
in rev c3f3393b9096 doesn't actually work since tests
introduced in mentioned changeset prevented dirstate
updates even if working directory updates succeded.
Make tests more relaxed for dirstate operations
not directly accessible from cli. See also issue660.
While here, move _dirs existance check from _decpath()
to _changepath() for unification.
Allow adding to dirstate files that clash with previously existing
but marked for removal. Protect from reintroducing clashes by revert.
This change doesn't address related issues with update. Current
workaround is to do "clean" update by manually removing conflicting
files/dirs from working directory.
