dirstate-v2: no longer register the data-file during transaction If the data file change during the transaction, we cannot truncate it. The content of the file itself is fine as it will get backed up at the same time as the docket. Leaving the trailing data at the end of failed transaction is fine. The dirstate-v2 format supports it. The dead data will simply we written over if necessary.

local test passed
addgroupcopy test passed
clone test passed
lowlevelcopy test passed
slicing test passed
issnapshot test passed
findsnapshot test passed