pkg-merge-upstream-to-downstreams: Ensure local downstream branch is created

In some cases the current code ensures that a local downstream branch,
for instance `apertis/v2021` or `apertis/v2021-updates`, is available if
a corresponding ref exists in the `origin` remote.

However, in some other cases the local ref is not created, creating an
annoying inconsistency. For instance it is never created for development
branches like `apertis/v2022dev1`. Even for stable branches, when
merging from a security branch like `debian/buster-security` and a
`origin/apertis/v2021-security` remote ref is available, the local
`apertis/v2021-security` ref is not created.

Tweak the code to always ensure there's a local ref for the selected
downstream branch.

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