test-ctype: check EOF

The character classifiers are supposed to allow passing EOF to them, a
negative value.  It isn't part of any character class.  Extend the tests
to cover that.

Signed-off-by: René Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/helper/test-ctype.c

@@ -28,6 +28,8 @@ static int is_in(const char *s, int ch)
 		if (is_in(s, i) != t(i))	\
 			report_error(#t, i);	\
 	}					\
+	if (t(EOF))				\
+		report_error(#t, EOF);		\
 }
 
 #define DIGIT "0123456789"