t5100: Avoid filename "nul"

There are broken filesystems that cannot have a file whose name is "nul"
anywhere on it.  Rename the test file to make ourselves more portable.

Noticed by Mark Levedahl.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t5100-mailinfo.sh

@@ -27,8 +27,8 @@ done
 
 test_expect_success 'respect NULs' '
 
-	git mailsplit -d3 -o. ../t5100/nul &&
+	git mailsplit -d3 -o. ../t5100/nul-plain &&
-	cmp ../t5100/nul 001 &&
+	cmp ../t5100/nul-plain 001 &&
 	(cat 001 | git mailinfo msg patch) &&
 	test 4 = $(wc -l < patch)