diff: make --stat respect ui.relative-paths

Authored by martinvonz.


diff: make --stat respect ui.relative-paths

It would have been easy to make all diffs respect ui.relative-paths,
but we don't want that since it makes the diff invalid. Perhaps it
makes sense to do that with --noprefix since the point of that is to
make paths that are easy to copy&paste, and the diff is already
invalid anyway. But this patch just makes the --stat version respect
the config option. The --stat view is not even close to a valid diff,
so I think it makes sense to show the paths in more human-friendly

Differential Revision: https://phab.mercurial-scm.org/D5896