t/lib-credential: use write_script

Use write_script to create the helper "askpass" script, instead of
hand-creating it with hardcoded "#!/bin/sh" to make sure we use the
shell the user told us to use.

Signed-off-by: Ben Walton <bdwalton@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/lib-credential.sh

@@ -278,12 +278,10 @@ helper_test_timeout() {
 	'
 }
 
-cat >askpass <<\EOF
-#!/bin/sh
+write_script askpass <<\EOF
 echo >&2 askpass: $*
 what=$(echo $1 | cut -d" " -f1 | tr A-Z a-z | tr -cd a-z)
 echo "askpass-$what"
 EOF
-chmod +x askpass
 GIT_ASKPASS="$PWD/askpass"
 export GIT_ASKPASS