new script: delta-all-the-things

[cgwalters]

Following the path of having ostree be a lower level cmdline, I'd like to propose a higher level script here which helps OS vendors manage static deltas.

I'm thinking something like this:

$ cat deltas.yml
deltas:
  # This is the stable branch created using do-release-tags, assuming a cadence of ~2 weeks, we want to support direct upgrades from any release in N-4 (i.e. ~2.5 months) to be relatively conservative
  - fedora/26/x86_64/stable/atomic-host:
      direct-depth: 5
  # CI/CD branch - we want a regular delta from N-1 to N, as well as deltas both from *and* to this and the stable branch above
  - fedora/26/x86_64/alpha/atomic-host:
       direct-depth: 0
      bidirectional: fedora/26/x86_64/stable/atomic-host
        depth: 0

Likely to start we'd just do the direct-depth for a single branch.