t4034: fix use of one-shot variable assignment with shell function

t4034: fix use of one-shot variable assignment with shell function

The behavior of a one-shot environment variable assignment of the form
"VAR=val cmd" is unspecified according to POSIX when "cmd" is a shell
function. Indeed the behavior differs between shell implementations and
even different versions of the same shell, thus should be avoided.

Signed-off-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>