Backport 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