Support "log.showSignature = true"

Support "log.showSignature = true"

Don't let "log.showSignature = true" in global git config cause
signature check output to appear in the release name.

Bad excuses for not including a test:
  * Would have to add GPG as a dependency.
  * Generating a GPG key during the test would be slow.
  * Making a local "global" git config in the test would be troublesome.