lib/commit: Implement "adoption" with CONSUME flag

lib/commit: Implement "adoption" with CONSUME flag

For checkouts that are on the same device, for regular files we can simply
"adopt" existing files. This is useful in the "build from subtrees" pattern that
happens with e.g. `rpm-ostree install` as well as flatpak and gnome-continuous.

New files are things like an updated `ldconfig` cache, etc. And particularly for
`rpm-ostree` we always regenerate the rpmdb, which for e.g. this workstation is
`61MB`.

We probably should have done this from the start, and instead had a `--copy`
flag to commit, but obviously we have to be backwards compatible.

There's more to do here - the biggest gap is probably for `bare-user` repos,
which are often used with things like `rpm-ostree compose tree` for host
systems. But we can do that later.

Closes: #1272
Approved by: jlebon