environment: normalize use of prefixcmp() by removing " != 0"

To be able to automatically convert prefixcmp() to starts_with()
we need first to make sure that prefixcmp() is always used in
the same way.

So let's remove " != 0" after prefixcmp().

Signed-off-by: Christian Couder <chriscool@tuxfamily.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>