fsmonitor: use new dirstate APIs (issue6728) On top of fixing fsmonitor, it moves one more "old API" use to the new one. This needs very verbose code to save a few function calls that are very expensive in Python.

dirstate-v2: backup the data file during the transaction (issue6730) If we backup of the docket, without doing a backup of the data file, we highly risk restoring a docket pointing to a missing file in the future. So we now backup the data-file alongside the docket.

dirstate-v2: display a possible issue with transaction If we backup the dirstate without the data file, we can end up in an inconsistent state of the transaction is rolled back.

test-dirstate: do not get out of the test directory It looks like we were doing one `cd ..` too many.