t6050: use git-update-ref rather than filesystem access

Signed-off-by: Han-Wen Nienhuys <hanwen@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t6050-replace.sh

@@ -132,7 +132,7 @@ tagger T A Gger <> 0 +0000
 EOF
 
 test_expect_success 'tag replaced commit' '
-     git mktag <tag.sig >.git/refs/tags/mytag
+     git update-ref refs/tags/mytag $(git mktag <tag.sig)
 '
 
 test_expect_success '"git fsck" works' '