t3404: make test 57 work with dash and others

t3404: make test 57 work with dash and others

The construct

	VAR=value test_must_fail command args

works only for some shells (such as bash) but not others (such as dash)
because VAR=value does not end up in the environment for command when it
is called by the shell function test_must_fail. That is why we explicitly
set and export variable in a subshell, i.e.

	(
		VAR=value &&
		export VAR &&
		test_must_fail command args
	)

in most places already, bar the newly introduced 57 from b64b7fe
(Add tests for rebase -i --root without --onto, 2012-06-26).

Make test 57 use that construct also.

Signed-off-by: Michael J Gruber <git@drmicha.warpmail.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>