Moving existing ostree users to a new branch

[Daniel Drake <drake endlessm com>]

Hi,

At Endless we are facing the challenge of both having to provide a
stable, well tested software distribution, while simultaneously having
to support the very latest hardware that requires us to pull
lesser-tested code from other project's git master branches,
introducing risk of instability for all.

To solve this, we are taking the approach of having 2 ostree branches:
 1. Stable, tested, trusted main branch
 2. Modified branch with cutting-edge code for brand new hardware support

Branch #2 will only be provided to the subset of users who have such
new hardware, and they will be automatically migrated to branch #1 at
a relevant point in future when releases iterate and the corresponding
hardware support becomes present in the main branch.

We now need to implement this automatic migration. I understand there
have been some previous conversations touching on the possibility of
having a new commit type which marks a specific branch as end-of-life.
I now plan to implement such functionality. I will aim for inclusion
upstream once the initial implementation is ready, so I would
appreciate any quick design guidance before I get into it.

My thinking is that this new commit type would also point to a new
branch that the client should switch to. When doing an ostree update
that ends up on this type of commit, the client would then update the
local repo config to point at the new branch. Then it would restart
the update process using the new branch.

Thanks for any input!

Daniel