lib-patch-mode tests: change from skip_all=* to prereq skip

Change this test to skip test with test prerequisites, and to do setup
work in tests. This improves the skipped statistics on platforms where
the test isn't run.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/lib-patch-mode.sh

@@ -2,11 +2,6 @@
 
 . ./test-lib.sh
 
-if ! test_have_prereq PERL; then
-	skip_all='skipping --patch tests, perl not available'
-	test_done
-fi
-
 set_state () {
 	echo "$3" > "$1" &&
 	git add "$1" &&