generate-git-snapshot: support git-buildpackage >=0.6.24
generate-git-snapshot: support git-buildpackage >=0.6.24

Since git-buildpackage v0.6.24 the `git-buildpackage` and
`git-dch` commands no longer exist, instead we need to use
`gbp buildpackage` and `gbp dch`.

Closes: #799183
Thanks: Daniele E. Domenichelli <709972d94845efffd8a79a64d6e4ff92e80e388f@gmail.com> for the bug report

tonin/jenkins-debian-glue,kartikm/jenkins-debian-glue,mika/jenkins-debian-glue,wikimedia/operations-debs-jenkins-debian-glue,linuxmaniac/jenkins-debian-glue,linuxmaniac/jenkins-debian-glue,hashar/jenkins-debian-glue,hashar/jenkins-debian-glue,webtrekk-kane/jenkins-debian-glue,webtrekk-kane/jenkins-debian-glue,bmiklautz/jenkins-debian-glue,kartikm/jenkins-debian-glue,wikimedia/operations-debs-jenkins-debian-glue,bmiklautz/jenkins-debian-glue,jbfavre/jenkins-debian-glue,jbfavre/jenkins-debian-glue,mika/jenkins-debian-glue,tonin/jenkins-debian-glue