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>