gpg-agent warning looks a little messy when inputting passphrase on terminal

When signing a message whilst logged in over ssh without gpg-agent available, the warning output from keybase client (I think) looks a little messy on the terminal because it's splodged in with the passphrase prompt:

```
$ keybase sign -m 'Hello'
You need a passphrase to unlock the secret key for
user: "David Wagner (A.K.A. mnem, mnemosyne, kaeladan) "
3072-bit DSA key, ID 0xC254EFC18EC75D35, created 2012-11-21

Enter passphrase: warn: gpg: gpg-agent is not available in this session
```

I imagine it looks similar for other actions I need to input my passphrase for.

Reproduce `keybase sign -m 'Hello'` over SSH without gpg-agent available and observe how the warning is interleaved with the passphrase prompt. Trace the signing and passphrase-input entry points to identify the output source; done means the warning is presented cleanly without corrupting the prompt.

Written by the indexing model from the issue text.