git-p4: remove bash-ism in t9800

git-p4: remove bash-ism in t9800

This works in both bash and dash:

    $ bash -c 'VAR=1 env' | grep VAR
    VAR=1
    $ dash -c 'VAR=1 env' | grep VAR
    VAR=1

But environment variables assigned this way are not necessarily propagated
through a function in POSIX compliant shells:

    $ bash -c 'f() { "$@"
    }; VAR=1 f "env"' | grep VAR
    VAR=1
    $ dash -c 'f() { "$@"
    }; VAR=1 f "env"' | grep VAR

Fix constructs like this, in particular, setting variables through
test_must_fail.

Based-on-patch-by: Vitor Antunes <vitor.hda@gmail.com>
Signed-off-by: Pete Wyckoff <pw@padd.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>