fsmonitor: rename new verbose config knob
The config knob was introduced in this release cycle under the old extension
name, rename it before it is part of a release.
Differential Revision: https://phab.mercurial-scm.org/D5880
#
# Avoid interference from actual test env:
unset HGTEST_JOBS
unset HGTEST_TIMEOUT
unset HGTEST_PORT
unset HGTEST_SHELL