evolve: fix the dirstate after setting parents with dirty wdir (issue5930)
Before this patch, during an interrupted evolve it can be confused
about copies. This patch fix the dirstate when parents are changed
using dirstate.setparents() with a dirty wdir.
Changes in test file reflect the fixed behaviour.
evolve: add tests which shows evolve can get confused about copies (issue5930)
This test demonstrate that an interrupted evolve can get confused about
copies. Next patch will fix this.