can't enter passphrase using gpg2 when signing imported public key

```
$ echo '{"body":{"key":{"fingerprint":"42725dc053624f593cedccd7904f63306ae405c8","host":"keybase.io","key_id":"904f63306ae405c8","uid":"1e6100566ef04c0fc7ca9420a1327a00","username":"ilia"},"type":"web_service_binding","version":1},"ctime":1394075926,"expire_in":157680000,"prev":null,"seqno":1,"tag":"signature"}' | gpg2 -u '904F63306AE405C8' -a --sign

You need a passphrase to unlock the secret key for
user: "Ilia Lobsanov "
1024-bit DSA key, ID 6AE405C8, created 2001-02-28

pinentry-gtk2: no LC_CTYPE known - assuming UTF-8
pinentry-gtk2: no LC_CTYPE known - assuming UTF-8
pinentry-gtk2: no LC_CTYPE known - assuming UTF-8
pinentry-gtk2: no LC_CTYPE known - assuming UTF-8

gpg: signal Interrupt caught ... exiting
```

Start by reproducing the shown gpg2 command and observe the pinentry-gtk2 messages and interrupted passphrase prompt. Investigate how the keybase client invokes gpg2 and pinentry for signing imported keys. Done means the command accepts the passphrase and completes signing without interruption.

Written by the indexing model from the issue text.