Fix false positives in t3404 due to SHELL=/bin/false

Fix false positives in t3404 due to SHELL=/bin/false

If the user's shell in NSS passwd is /bin/false (eg as found during Gentoo's
package building), the git-rebase exec tests will fail, because they call
$SHELL around the command, and in the existing testcase, $SHELL was not being
cleared sufficently.

This lead to false positive failures of t3404 on systems where the package
build user was locked down as noted above.

Signed-off-by: "Robin H. Johnson" <robbat2@gentoo.org>
X-Gentoo-Bug: 349083
X-Gentoo-Bug-URL: http://bugs.gentoo.org/show_bug.cgi?id=349083
Signed-off-by: Junio C Hamano <gitster@pobox.com>