### Please describe the problem.
git-annex refuses to use revoked GPG keys. This may be understandable for the initial remote setup, but it hit me when I tried to add a new key to a remote. The previous key has been revoked (because it has been superseded by the new one), and git-annex refused to reinvoke the shared key with both keys because one of them was revoked.

Given the encryption model does not allow key replacement, it should not refuse to reencrypt using a revoked key. Maybe using `--expert` would help.

### What steps will reproduce the problem?
Encrypt a special remote with a key K1. Revoke key K1. Try to add key K2 with enableremote. git-annex will refuse to encrypt the shared key with the revoked one.

### What version of git-annex are you using? On what operating system?
git-annex version: 4.20130802-g1452ac3

### Please provide any additional information below.

[[!format sh """
% git annex enableremote zoidberg-crypted encryption=42B8F7C2 
enableremote zoidberg-crypted (encryption update) 
You need a passphrase to unlock the secret key for
user: "Samuel Tardieu <sam@rfc1149.net>"
2048-bit ELG key, ID F0D70BAF, created 2002-05-31 (main key ID 1B80ADE6)

gpg: NOTE: key has been revoked
gpg: reason for revocation: Key is superseded
gpg: revocation comment: Key superseded by 42B8F7C2
gpg: revocation comment: (fingerprint 1D36 D924 8B33 DCAB 7BA5  BA44 7A30 BCF4 42B8 F7C2)
gpg: F13322411B80ADE6: skipped: Unusable public key
gpg: [stdin]: encryption failed: Unusable public key

git-annex: user error (gpg ["--quiet","--trust-model","always","--encrypt","--no-encrypt-to","--no-default-recipient","--recipient","7A30BCF442B8F7C2","--recipient","F13322411B80ADE6"] exited 2)
failed
git-annex: enableremote: 1 failed
"""]]

> [[done]]; can now use: `git annex enableremote foo keyid-=REVOKEDKEY
> keyid+=NEWKEY` to remove it, and add a new key. --[[Joey]]  
