Checkout the Debian branch before importing sources

Signed-off-by: Andrej Shadura <andrew.shadura@collabora.co.uk>

apertis-package-source-builder/overlay/usr/bin/apertis-pkg-pull-updates

@@ -90,6 +90,7 @@ def get_remote_sources(remote_dsc, tmpdir):
   return os.path.join(tmpdir, os.path.basename(remote_dsc))
 
 def import_sources(local_dsc, upstream_suite):
+  run(['git', 'checkout', debian_branch(upstream_suite)])
   run(['gbp', 'import-dsc', local_dsc, '--debian-branch=' + debian_branch(upstream_suite), '--upstream-branch='+upstream_branch(upstream_suite), '--no-pristine-tar',
        '--author-is-committer', '--author-date-is-committer-date', '--no-sign-tags'
       ],