"not in cache" error from `git ls-files -m | git checkout-index -f --stdin` ?

Hi, it seems to me that in this sequence:

    git ls-files -m | git checkout-index --stdin -m

c-i should never complain that a file isn't in the cache, but that's
exactly what this script produces for me:

-8<----------

#!/bin/sh
rm -rf reproduce-c-i-weirdity
git init --template= reproduce-c-i-weirdity
cd reproduce-c-i-weirdity

mkdir content
mkdir deployment

echo '*/' >>.gitignore
echo '!content/' >>.gitignore

echo deployme >>content/deployme

git add .
git commit -m-

git init --template= --bare test.git
git remote add test $PWD/test.git

mkdir test.git/hooks
cat >test.git/hooks/pre-receive <<\EOD
#!/bin/sh -x
while read old new ref; do [[ $ref = refs/heads/master ]] && {
	export GIT_WORK_TREE=../deployment
	export GIT_INDEX_FILE=deployment-manifest
	git read-tree -um `git write-tree` $new:content \
	|| { git ls-files -m | tee /dev/stderr | git checkout-index -f --stdin;
		git read-tree -um `git write-tree` $new:content; } \
	|| exit $?
}
done
EOD
chmod +x test.git/hooks/pre-receive

git push test master

echo nonono > deployment/deployme
echo yesyes > content/deployme

git commit -am-
git push test master

->8-------------------

read-tree and ls-files can both find the file and see that it's changed,
so why not checkout-index?