bin/debpkg-bootstrap-branches: Fix idempotency call
Without this, the debian/upstream branch was not really dropped. Failing to repeat the
complete cycle.
... + git checkout v0.1.0 HEAD is now at 04b9a34 Add a non-optimized non-incremental Bazaar loader + git clean -dfx Removing .eggs/ Removing build/ Removing swh.loader.bzr.egg-info/ + git checkout -b debian/upstream fatal: A branch named 'debian/upstream' already exists.
Related to T3915