fsmonitor: execute setup procedures only if dirstate is already instantiated
Before this patch, reposetup() of fsmonitor executes setup procedures
for dirstate, even if it isn't yet instantiated at that time.
On the other hand, dirstate might be already instantiated before
reposetup() intentionally (prefilling by chg, for example, see
bf3af0eced44 for detail). If so, just discarding already instantiated
one in reposetup() causes issue.
To resolve both issues above, this patch executes setup procedures,
only if dirstate is already instantiated.
BTW, this patch removes "del repo.unfiltered().__dict__['dirstate']",
because it is responsibility of the code path, which causes
instantiation of dirstate before reposetup(). After this patch, using
localrepo.isfilecached() should avoid creating the corresponded entry
in repo.unfiltered().__dict__.