Uncaught ValueError when tags are not pulled into local repository

Example to reproduce the issue:

git clone https://gitlab.com/buildstream/buildstream.git --no-tags
cd buildstream
python3 setup.py test --addopts "tests/sources/zip.py::test_no_ref"

Here is an extract of the stack we get:

  File "/tmp/buildstream/buildstream/_plugincontext.py", line 190, in _load_plugin
    self._assert_version(kind, plugin_type)
  File "/tmp/buildstream/buildstream/_plugincontext.py", line 212, in _assert_version
    bst_major, bst_minor = utils.get_bst_version()
  File "/tmp/buildstream/buildstream/utils.py", line 475, in get_bst_version
    return (int(versions[0]), int(versions[1]))
ValueError: invalid literal for int() with base 10: '0+untagged'

This issue can happen to people who have done only git pull origin master since before there were tags.

To work around this problem, run git fetch.