Deleted files are being restored to the working tree

When I tried updating to 0.3.4, the pre-commit hook started restoring files I had deleted.

Using `git stash push --keep-index` as in #30 restores deleted files to the working tree:
```
$ git init
Initialized empty Git repository in /tmp/git-stash-test/.git/
$ touch test
$ git add -A
$ git commit -m 'initial commit'
[master (root-commit) 5e0577e] initial commit
1 file changed, 0 insertions(+), 0 deletions(-)
create mode 100644 test
$ rm test
$ git add -A
$ git status
On branch master
Changes to be committed:
(use "git reset HEAD ..." to unstage)

deleted: test

$ git stash push --keep-index --message pre_commit
Saved working directory and index state On master: pre_commit
$ git stash pop
On branch master
Changes to be committed:
(use "git reset HEAD ..." to unstage)

deleted: test

Untracked files:
(use "git add ..." to include in what will be committed)

test

Dropped refs/stash@{0} (b6658734f4ae35a43bcff3f43b306ea6ae238902)
```