git-crypt unlock not working

Hi,

I try to decrypt a git-crypted file outside of git. We use symmetric en-/decryption with different keys.
.gitattributes contains:
```
*dev_test.properties.enc filter=git-crypt-dev_test diff=git-crypt-dev_test
*dev_live.properties.enc filter=git-crypt-dev_live diff=git-crypt-dev_live
*prod_live.properties.enc filter=git-crypt-prod_live diff=git-crypt-prod_live
```
Folder looks like:
```
|-.gitattributes
|-application_credential_dev_live.properties.enc
|-application_credential_dev_test.properties.enc
|-application_credential_prod_live.properties.enc
```
I try to decrypt lets say the dev_test file with

```
git init
git-crypt unlock dev_test.key
```

As result I get:
```
fatal: You are on a branch yet to be born
Error: 'git checkout' failed
git-crypt has been set up but existing encrypted files have not been decrypted
```

I've executed `openssl dgst -sha256 .git/git-crypt/keys/dev_test` on both sides, the sh256 is the same ... do you've any idea?