verify-tag: Parse GPG configuration options.

Modify verify-tag to load relevant GPG variables from the git
configuratio file.  This allows git tag -v to use an alternative
GPG binary in the same way that git tag -s does.

Signed-off-by: Alex Zepeda <alex@inferiorhumanorgans.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>