tools/pipbot: Show last release version in burndown

This transforms:

    $ pipbot burndown release
    Comparing 'master' against 'stable'...

      There are 9 new commits
      Last commit was 47 minutes ago

Into:

    $ pipbot burndown release
    Comparing 'master' against 'stable'...

      The last release was 0.1.7
      There are 9 new commits
      Last commit was 47 minutes ago
tools/pipbot: Show branch names on burndown

This transforms:

    $ pipbot burndown release
    Comparing 'master' against 'stable'...

      The last release was 0.1.7
      There are 9 new commits
      Last commit was 47 minutes ago

Into:

    $ pipbot burndown release
    Comparing 'master' against 'stable'...

      The last release was 0.1.7
      There are 9 new commits on master
      Last commit on stable was 47 minutes ago