Support deployment garbage collection based on versions

[jlebon]

This came up in ostreedev/ostree#1922. Essentially, the idea is to make garbage collection smarter by always keeping at least the last two versions of deployments. This prevents repeated layered commits (from e.g. rpm-ostree install commands) from flushing out the previous version's deployment.

The rationale is that for e.g. Silverblue/FCOS systems, the version string is associated with a specific snapshot of Fedora content. So in general, they're more meaningful deltas in terms of fallback than local recomposes from install operations.

[jlebon]

I filed this here, but I think most of the work for this would take place in libostree. E.g. have a new OstreeSysrootSimpleWriteDeploymentFlags flag like OSTREE_SYSROOT_SIMPLE_WRITE_DEPLOYMENT_FLAGS_RETAIN_PREVIOUS_VERSION? Though offhand, looks like at least this TODO would have to be addressed so that the staging path (which is the default now) can pass that flag along.