pkg-merge-upstream-to-downstreams: Explicitly define stable branches

Currently the code pulling updates from upstream assumes that downstream
branches ending with `dev[0-9]` or `pre` are not stable so updates can
be directly landed there, while other downstream branches (for instance,
`apertis/v2021`) are stable and updates should go through the updates or
security repositories.

However, there's a time right before the first stable release is
published that the "stable" branch is not stable yet. For instance,
before the release of `v2021.0` the `apertis/v2021` should not have been
considered stable and updates should have been landed directly there.

This is even more important for downstreams so that they can pull
updates to their not-released-yet v2021 branch.

To do so, add a `--stable` parameter that accepts the list of branches
to be considered "stable".

Signed-off-by: Emanuele Aina <emanuele.aina@collabora.com>