generate-git-snapshot: make sure we're building the requested version via dpkg-source

If we are changing back to the previous branch before running dpkg-source
then we always build the latest version instead of the requested one.
So make sure to actually generate the tarballs and *then* switch back
to the original GIT_BRANCH.
generate-git-snapshot: support $branch as alternative to $tag

When switching from svn to git the configuration might include
the "branch" parameter for the Jenkins build, easify migration
path.