The previous commit to dynamically detect the default branch
name (instead of assuming 'master') only worked if the fork
repo used the same default branch name as upstream. This is
only likely to be the case for forks created after the
upstream branch rename, and even then developers might change
their fork's default branch name.

Fortunately 'git remote show' prints info including the
default branch name:

$ git remote show check-dco
* remote check-dco
  Fetch URL: https://gitlab.com/libosinfo/osinfo-db.git
  Push  URL: https://gitlab.com/libosinfo/osinfo-db.git


  HEAD branch: main
  Remote branch:
    main new (next fetch will store in remotes/check-dco)

Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>