rust-dirstate-v2: clean up previous data file after the docket is written
This was overlooked before and caused many data files to keep living forever.
We could potentially consider adding a random cleanup of them in case they
slipped through one day.
Differential Revision: https://phab.mercurial-scm.org/D12581
edition = "2018"
max_width = 79
wrap_comments = true
error_on_line_overflow = true