t7502: test early quit from commit with bad ident

t7502: test early quit from commit with bad ident

In commit f20f387, "git commit" notices and dies much
earlier when we have a bogus commit identity. That commit
did not add a test because we cannot do so reliably (namely,
we can only trigger the behavior on a system where the
automatically generated identity is bogus). However, now
that we have a prerequisite check for this feature, we can
add a test that will at least run on systems that produce
such a bogus identity.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>