Doesn't work with sparse checkouts.

## Background

I have a big monorepo with a lot of files, some of which are git-crypted. In a CI server I need to check out a submodule (using a git sparse checkout) and decrypt the included subset of the git-crypted files. Git-crypt won't allow this.

## Reproduction

(using bash)

```bash
mkdir -p git-crypt-sparse-demo/repo1/foo \
git-crypt-sparse-demo/repo1/bar \
git-crypt-sparse-demo/repo2

# set up a test repository (repo1) with 2 encrypted files
cd git-crypt-sparse-demo/repo1
git init
git-crypt init
# NOTE: You'll need to replace me@example.com below with a real gpg user from your keychain
git-crypt add-gpg-user me@example.com
echo "foo" >> foo/foo.txt && echo "bar" >> bar/bar.txt
echo -e "/foo/foo.txt filter=git-crypt diff=git-crypt\n/bar/bar.txt filter=git-crypt diff=git-crypt" >> .gitattributes
git add .
git commit -m "foo and bar"
git-crypt lock

# set up a sparse checkout of repo1, in the repo2 folder, that omits everything under /bar
cd ../repo2
git init
git config core.sparseCheckout true
git remote add origin "file://`pwd`/../repo1"
echo "/foo/" >> .git/info/sparse-checkout
echo "/.gitattributes" >> .git/info/sparse-checkout
echo "/.git-crypt" >> .git/info/sparse-checkout
git fetch --depth 1 origin
git checkout master
# now we've got foo/ checked out, try to unlock foo/foo.txt
git-crypt unlock
```

## Expected Behaviour

`foo/foo.txt` is decrypted

## Actual Behaviour

`foo/foo.txt` is still encrypted, and the following error message is displayed:

```
error: pathspec 'bar/bar.txt' did not match any file(s) known to git.
Error: 'git checkout' failed
git-crypt has been set up but existing encrypted files have not been decrypted
```