Consider removing dependency on external GPG installation for CreateCommitSignature
As referenced in gitlab-com/gl-infra/scalability#259 (comment 330564176).
The concern is that using_tmp_keychain is single-threaded and this may be causing the create_commit_signature worker to saturate its queue. This issue is intended to explore the possibility of getting rid of the GPGME
dependency.
@nick.thomas has suggested that https://godoc.org/golang.org/x/crypto/openpgp may be enough to perform the operations needed.