tests/test-gendoc-zh_CN.t

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

#require docutils gettext

  $ $TESTDIR/check-gendoc zh_CN
  checking for parse errors