Multiline comment in a "(>)" crashes "bst track"

Summary

Multiline comments

Steps to reproduce

Try to track the following element:

kind: manual

config:
  build-commands:
    (<):
    # BuildStream does not like that
    # multiline comment
    - |
      true

sources:
- kind: remote
  url: https://www.timeanddate.com/worldclock/uk/manchester

What is the current bug behavior?

        Traceback (most recent call last):
          File "/home/valentin/.local/lib/python3.7/site-packages/buildstream/_scheduler/queues/queue.py", line 280, in _job_done
            self.done(job, element, result, status)
          File "/home/valentin/.local/lib/python3.7/site-packages/buildstream/_scheduler/queues/trackqueue.py", line 60, in done
            source._set_ref(new_ref, save=True)
          File "/home/valentin/.local/lib/python3.7/site-packages/buildstream/source.py", line 879, in _set_ref
            _yaml.dump(provenance.toplevel, provenance.filename.name)
          File "/home/valentin/.local/lib/python3.7/site-packages/buildstream/_yaml.py", line 248, in dump
            yaml.round_trip_dump(node, f)
          File "/home/valentin/.local/lib/python3.7/site-packages/ruamel/yaml/main.py", line 1312, in round_trip_dump
            prefix_colon=prefix_colon,
          File "/home/valentin/.local/lib/python3.7/site-packages/ruamel/yaml/main.py", line 1190, in dump_all
            dumper._representer.represent(data)
          File "/home/valentin/.local/lib/python3.7/site-packages/ruamel/yaml/representer.py", line 85, in represent
            node = self.represent_data(data)
          File "/home/valentin/.local/lib/python3.7/site-packages/ruamel/yaml/representer.py", line 112, in represent_data
            node = self.yaml_representers[data_types[0]](self, data)
          File "/home/valentin/.local/lib/python3.7/site-packages/ruamel/yaml/representer.py", line 1173, in represent_dict
            return self.represent_mapping(tag, data)
          File "/home/valentin/.local/lib/python3.7/site-packages/ruamel/yaml/representer.py", line 1018, in represent_mapping
            node_value = self.represent_data(item_value)
          File "/home/valentin/.local/lib/python3.7/site-packages/ruamel/yaml/representer.py", line 112, in represent_data
            node = self.yaml_representers[data_types[0]](self, data)
          File "/home/valentin/.local/lib/python3.7/site-packages/ruamel/yaml/representer.py", line 1173, in represent_dict
            return self.represent_mapping(tag, data)
          File "/home/valentin/.local/lib/python3.7/site-packages/ruamel/yaml/representer.py", line 1018, in represent_mapping
            node_value = self.represent_data(item_value)
          File "/home/valentin/.local/lib/python3.7/site-packages/ruamel/yaml/representer.py", line 112, in represent_data
            node = self.yaml_representers[data_types[0]](self, data)
          File "/home/valentin/.local/lib/python3.7/site-packages/ruamel/yaml/representer.py", line 1173, in represent_dict
            return self.represent_mapping(tag, data)
          File "/home/valentin/.local/lib/python3.7/site-packages/ruamel/yaml/representer.py", line 1018, in represent_mapping
            node_value = self.represent_data(item_value)
          File "/home/valentin/.local/lib/python3.7/site-packages/ruamel/yaml/representer.py", line 112, in represent_data
            node = self.yaml_representers[data_types[0]](self, data)
          File "/home/valentin/.local/lib/python3.7/site-packages/ruamel/yaml/representer.py", line 1188, in represent_list
            return self.represent_sequence(tag, data)
          File "/home/valentin/.local/lib/python3.7/site-packages/ruamel/yaml/representer.py", line 935, in represent_sequence
            self.merge_comments(node_item, item_comments.get(idx))
          File "/home/valentin/.local/lib/python3.7/site-packages/ruamel/yaml/representer.py", line 957, in merge_comments
            assert val is None or val == nc
        AssertionError

What is the expected correct behavior?

It should just track and succeed.

Relevant logs and/or screenshots

Possible fixes

Other relevant information