RuntimeError: Hit unreachable condition on 'tree-reference' directory entry kind

Sentry Issue: SWH-LOADER-BZR-71

RuntimeError: Hit unreachable condition
  File "swh/loader/core/loader.py", line 414, in load
    self.store_data()
  File "swh/loader/bzr/loader.py", line 340, in store_data
    self.store_revision(self.repo.get_revision(rev))
  File "swh/loader/bzr/loader.py", line 396, in store_revision
    directory = self.store_directories(bzr_rev)
  File "swh/loader/bzr/loader.py", line 481, in store_directories
    content = self.store_content(bzr_rev, path, kind, executable, size)
  File "swh/loader/bzr/loader.py", line 529, in store_content
    raise RuntimeError("Hit unreachable condition")

Loading failure, updating to `failed` status

while loading https://code.launchpad.net/~serge-hallyn/ubuntu/precise/qemu-linaro/merge-spice

file_path is 'roms/pcbios' and kind is 'tree-reference', while processing bzr_rev = <Revision id b'git-v1:c34ebfdc8715f03d53e154758423c902324fee06'>