common-subtree: Fix branch to use

The `test/common` repository uses `apertis/v20*` branches, not `main` or
`master`.
`common-subtree.sh` is using the current branch from the working
test repo to infer which remote branch to import: if it's not in the
`apertis/*` form, retrieve the latest branch from the `test/common`
repository.

Signed-off-by: Frédéric Danis <frederic.danis@collabora.com>