[gpg] Correct finding public keys from pubring.gpg

[gpg] Correct finding public keys from pubring.gpg

With a master key not enabled for signing, and a signing sub-key, key
lookup went wrong in several ways and might not find a suitable key for
signing or for signature verification. Fix the code so that it finds
the sub-key, even if user.signingKey is specified not with a key ID but
with an with an e-mail. (Sub-keys don't have user ids, those are
attached only on the master key.)

Change-Id: I9d1f641c49b173d4daffb3fd2e74f5aabd856e39
Signed-off-by: Thomas Wolf <twolf@apache.org>