darwin: add another preemptive gui() call when using chg
Changeset a89381e04c58 added this gui() call before background forks, and
Google's extensions do background forks on essentially every invocation for
logging purposes. The crash is reliably (though not 100%) reproducible without
this change when running `HGPLAIN=1 chg status` in one of our repos. With this
fix, I haven't been able to trigger the crash anymore.
Differential Revision: https://phab.mercurial-scm.org/D8141