dirstate: force _checkexec to return a bool
posix.checkexec can return True, False, or None. The rust status
implementation expects a boolean, so make sure _checkexec returns a boolean.
Differential Revision: https://phab.mercurial-scm.org/D8432
#
# Avoid interference from actual test env:
unset HGTEST_JOBS
unset HGTEST_TIMEOUT
unset HGTEST_PORT
unset HGTEST_SHELL