During the major upgrade of the Apertis OS with static delta
we need to use the commit containing current branch name in
metadata. Such commit would be accepted by AUM and libostree,
so the updated OS will be booted with new major version.

Due the nature of offline upgrade the branch name didn't changed
to the new name and AUM should to care about refs update.

In case if the system has been booted well we mark the update as
successful. Additional step has been added for marking the update
as complete -- read branches names from the commit metadata and
set repository to the newest version of Apertis if needed.
After this step the ostree repository will be configured for new
branch (release) usage.

Since all checks are done on the recent Apertis version during the
boot attempt, there is no need to change anything on the old system.

In case if refs could not be updated -- the update wouldn't be marked
as successful and rollback mechanism will boot the OS into previous state.

Signed-off-by: Denis Pynkin <denis.pynkin@collabora.com>