Work around #793471 (madness with $SIG{__WARN__} and Perl's system builtin): move...

Work around #793471 (madness with $SIG{__WARN__} and Perl's system builtin): move $SIG{} setting into setup_sigwarn in Dgit.pm, and check getppid.

We also need to fix up the expectation for an error message in one of
the tests, where the message changes as a side effect.