generate-git-snapshot: new versioning schema, including timestamp
generate-git-snapshot: new versioning schema, including timestamp

When moving Debian package builds from one CI system to another one
then the build number (BUILD_NUMBER) won't necessarily be the same
anymore. So do the same as in generate-svn-snapshot: include the
timestamp in the version string.

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