userdiff tests: explicitly test "default" pattern

Since 122aa6f9c0 (diff: introduce diff.<driver>.binary, 2008-10-05)
the internals of the userdiff.c code have understood a "default" name,
which is invoked as userdiff_find_by_name("default") and present in
the "builtin_drivers" struct. Let's test for this special case.

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

t/t4018-diff-funcname.sh

@@ -49,6 +49,7 @@ diffpatterns="
 	ruby
 	rust
 	tex
+	default
 	custom1
 	custom2
 	custom3