When checking builds, extract the spec file and verify Name: is set set correctly
    
    See the inline commit for an explanation.
    
    v2:
    - previously, when the spec file was not found in a commit, the script
      returned an error. But this causes the following case to fail:
      $ scripts/distgit-commit-unused.py --repository ~/fedora/buildbot origin/epel8
      because epel8 branch is forked directly from the initial commit and has no spec
      file. Trying to build from that commit would fail. So let's just ignore such
      commits for the purposes of that check.