Fix artifact log

Description

It seems that our artifact log command has not been working since the artifact as a proto work landed.

This was because we were checking ~/.cache/buildstream/cas/refs/heads/ to see whether the logs were cached. This directory is no longer populated in the local cache. So this patch involves updating artifact log so that it uses the artifact proto to get the logs.

As a side note, we were not seeing our test failing due to the fact that test_artifact_log was comparing an empty string to other empty strings. This has also been fixed.