compose-tree: Fix location of rpmdb querying for caching

We were just repeatedly hitting the cache...oops.

Also add an assertion that the returned data isn't empty.
Add a "rebase" builtin

This is exactly the code from "ostree admin switch", except it's
called "rebase" because in the future it will also carry along any
locally layered packages.
rollback: Add comment
Prelimiary test command files