allow git-secret killperson to work even if there are multiple keys for a single email, so users can recover from this situation more easily

[joshrabinowitz]

Currently killperson will abort if it finds multiple keys in a git-secret repo that match a given email. See #633 for details about keys matching emails.

There's a case where users can tell the same email with different keys, and it puts the git-secret keychain in a state it cannot recover from by itself.

Because of the issues mentioned in #268, I think killperson should get a new -F option, which specifically will allow removing keys from the git-secret keychain even if there are multiple keys for a given email.

[joshrabinowitz]

Fixed as per #638