Right now, this filtering is done by dirstate.write using the time of
dirstate.write method call. However that filtering is done "too late"
It works "fine" as most command are "fast enough", and race rare enough.
We are about to change the mtime filtering logic in the dirstate to be more
accurate and reliable.
However hg update will still need such filtering (mostly because it is
actually quite racy, even with the existing filtering). So we explicitly
implement a similar logic here. Before removing the older one later in the