use git-client builder to retrieve a GitClient
and not be locked by a specific implementation
use restored showRevision. 
Less compact but more readable code
API backward compatibility fix
more flexible management for default git installation
[JENKINS-16849] disable isMailerPropertySet
need to be able to check Mailer.UserProperty#address is set without invoking MailAddressResolver
[maven-release-plugin] prepare release git-1.2.0