mkosi: Relax separate branch check in mkosi.sync

mkosi: Relax separate branch check in mkosi.sync

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.