rollback: display some graphlog before/after a test piece This make the situation clearer.

rollback: show that the safety works in a associated test That test section is checking that --force is overcoming the safety check, however we did not check that the safety properly detect the situation. We now do for clarity.

dirstate-guard: remove its usage in `backout` We can simply replace it with a transaction.

dirstate-guard: remove the usage in `import` It is now redundant with the transaction spawning the same scope.