generate-git-snapshot: do not increase package version if building for UNRELEASED
generate-git-snapshot: do not increase package version if building for UNRELEASED

This brings generate-git-snapshot closer to what generate-svn-snapshot
already provided, jey.

Thanks: Raphael Hertzog <ec67638a84dca5d1ffa2be696be98fba4d39bfb8@ouaza.com> for the suggestion

bmiklautz/jenkins-debian-glue,mika/jenkins-debian-glue,jbfavre/jenkins-debian-glue,tonin/jenkins-debian-glue,hashar/jenkins-debian-glue,cclausen/jenkins-debian-glue,bmiklautz/jenkins-debian-glue,cclausen/jenkins-debian-glue,hashar/jenkins-debian-glue,zeha/jenkins-debian-glue,n054/jenkins-debian-glue,hetznerZA/jenkins-debian-glue,kartikm/jenkins-debian-glue,linuxmaniac/jenkins-debian-glue,wikimedia/operations-debs-jenkins-debian-glue,zeha/jenkins-debian-glue,tonin/jenkins-debian-glue,vorlock/jenkins-debian-glue,n054/jenkins-debian-glue,jbfavre/jenkins-debian-glue,tanglu-org/jenkins-tanglu-buildkit,wikimedia/operations-debs-jenkins-debian-glue,webtrekk-kane/jenkins-debian-glue,webtrekk-kane/jenkins-debian-glue,linuxmaniac/jenkins-debian-glue,mika/jenkins-debian-glue,kartikm/jenkins-debian-glue,vorlock/jenkins-debian-glue,tanglu-org/jenkins-tanglu-buildkit,hetznerZA/jenkins-debian-glue,tanglu-org/jenkins-tanglu-buildkit