Fix second call to bloom-release on a svn repo

On multi-package SVN repositories, the first release works, but
I could consistently make the 2nd one fail.
While I have not investigated the reason why the git repo ends up being
dirty, running git clean *before* git rm fixes the problem.
Merge pull request #281 from po1/fix-svn-release

Fix second call to bloom-release on a svn repo