t7509: avoid direct file access for writing CHERRY_PICK_HEAD

Signed-off-by: Han-Wen Nienhuys <hanwen@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/t/t7509-commit-authorship.sh b/t/t7509-commit-authorship.sh
index ee6c474..d568593 100755
--- a/t/t7509-commit-authorship.sh
+++ b/t/t7509-commit-authorship.sh
@@ -147,7 +147,7 @@
 	test_tick &&
 	git commit -am "cherry-pick 1" --author="Cherry <cherry@pick.er>" &&
 	git tag cherry-pick-head &&
-	git rev-parse cherry-pick-head >.git/CHERRY_PICK_HEAD &&
+	git update-ref CHERRY_PICK_HEAD $(git rev-parse cherry-pick-head) &&
 	echo "This is a MERGE_MSG" >.git/MERGE_MSG &&
 	echo "cherry-pick 1b" >>foo &&
 	test_tick &&
@@ -162,7 +162,7 @@
 '
 
 test_expect_success '--reset-author with CHERRY_PICK_HEAD' '
-	git rev-parse cherry-pick-head >.git/CHERRY_PICK_HEAD &&
+	git update-ref CHERRY_PICK_HEAD $(git rev-parse cherry-pick-head) &&
 	echo "cherry-pick 2" >>foo &&
 	test_tick &&
 	git commit -am "cherry-pick 2" --reset-author &&