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
#require black $ cd $RUNTESTDIR/.. $ black --config=black.toml --check --diff `hg files 'set:**.py - mercurial/thirdparty/** - "contrib/python-zstandard/**"'`