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

hgext/fsmonitor/__init__.py

@@ -927,7 +927,7 b' def reposetup(ui, repo):'
             return
 
         try:
-            client = watchmanclient.client(repo.ui, repo._root)
+            client = watchmanclient.client(repo.ui, repo.root)
         except Exception as ex:
             _handleunavailable(ui, fsmonitorstate, ex)
             return