t5550: use write_script to generate post-update hook

The post-update hooks created in t5550-http-fetch-dumb.sh is missing the
"!#/bin/sh" line which can cause issues with portability.  Instead
create the hook using the 'write_script' function which includes the
proper "#!" line.

Signed-off-by: Brandon Williams <bmwill@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t5550-http-fetch-dumb.sh

@@ -20,8 +20,9 @@ test_expect_success 'create http-accessible bare repository with loose objects'
 	(cd "$HTTPD_DOCUMENT_ROOT_PATH/repo.git" &&
 	 git config core.bare true &&
 	 mkdir -p hooks &&
-	 echo "exec git update-server-info" >hooks/post-update &&
-	 chmod +x hooks/post-update &&
+	 write_script "hooks/post-update" <<-\EOF &&
+	 exec git update-server-info
+	EOF
 	 hooks/post-update
 	) &&
 	git remote add public "$HTTPD_DOCUMENT_ROOT_PATH/repo.git" &&