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
pager.pager=p0 # $TESTTMP/sysrc:4
ui.editor=e1 # $TESTTMP/userrc:2
pager.pager=p2 # $PAGER
ui.editor=e1 # $TESTTMP/userrc:2