t0003: call dd with portable blocksize

The command `dd bs=101M count=1` is not portable,
e.g. dd shipped with MacOs does not understand the 'M'.

Use `dd bs=1048576 count=101`, which achives the same, instead.

Signed-off-by: Torsten Bögershausen <tboegi@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t0003-attributes.sh

@@ -363,7 +363,7 @@ test_expect_success 'large attributes line ignores trailing content in tree' '
 
 test_expect_success EXPENSIVE 'large attributes file ignored in tree' '
 	test_when_finished "rm .gitattributes" &&
-	dd if=/dev/zero of=.gitattributes bs=101M count=1 2>/dev/null &&
+	dd if=/dev/zero of=.gitattributes bs=1048576 count=101 2>/dev/null &&
 	git check-attr --all path >/dev/null 2>err &&
 	echo "warning: ignoring overly large gitattributes file ${SQ}.gitattributes${SQ}" >expect &&
 	test_cmp expect err
@@ -391,7 +391,7 @@ test_expect_success 'large attributes line ignores trailing content in index' '
 
 test_expect_success EXPENSIVE 'large attributes file ignored in index' '
 	test_when_finished "git update-index --remove .gitattributes" &&
-	blob=$(dd if=/dev/zero bs=101M count=1 2>/dev/null | git hash-object -w --stdin) &&
+	blob=$(dd if=/dev/zero bs=1048576 count=101 2>/dev/null | git hash-object -w --stdin) &&
 	git update-index --add --cacheinfo 100644,$blob,.gitattributes &&
 	git check-attr --cached --all path >/dev/null 2>err &&
 	echo "warning: ignoring overly large gitattributes blob ${SQ}.gitattributes${SQ}" >expect &&