Not easy to update encryption subkey user uses.

I was using git crypt but recently added a new encryption subkey to my key. I tried to update the repo to add this key but found I was unable to do this easily. I tried the following and neither updated or added a new encrypted key to the repo.

``` bash
git crypt add-gpg-user kevincox@kevincox.ca
git crypt add-gpg-user $subkeyid
```

While I understand the first not working (that key is already added) the second should probably add the subkey (or update the existing to the subkey possibly). It would also be nice to have a warning when nothing is done because the key "already exists".

I was able to work around this by deleting the key (`rm .git-crypt/keys/default/0/E3943366624E7449B9B485AEC0758A3B34D52E74.gpg`) then re-adding myself (`git crypt add-gpg-user kevincox@kevincox.ca`).