fsmonitor: access repo.root
There is no repo._root. It looks like fsmonitor has
been busted since this access was introduced in
ab1900323b1 in July 2019!
Differential Revision: https://phab.mercurial-scm.org/D7207
$ hg init
$ echo This is file a1 > a
$ hg add a
$ hg commit -m "commit #0"
$ ls
a
$ echo This is file b1 > b
$ hg add b
$ hg commit -m "commit #1"
$ hg co 0
0 files updated, 0 files merged, 1 files removed, 0 files unresolved
B should disappear
$ ls
a