There's no guarantee that $GIT_BRANCH is always the same across
stable release branches and main. But we still need to make sure
we switch commits if we're switching between main and release branches.
To make this work, let's not check if the current HEAD commit is an
ancestor of the currently configured origin branch. Instead, let's
check if any of the origin branches contains the current HEAD commit.
This still makes sure that we won't switch commits if we're working
on a local branch while making sure that we do switch commits if we're
switching between main and stable release branches with different
git branches configured for the packaging repo checkout.
Now that mkosi can automatically pick up its main configuration from
a mkosi/ subdirectory if it exists and there is no configuration in the
top level directory, let's make use of it to reduce the amount of clutter
in the top level directory of the repository.
This will also make it easier to install the mkosi configuration files as
part of the testing packages later on.
