"A tool for editing version-control repository history reposurgeon enables risky operations that version-control systems don’t want to let you do, such as editing past comments and metadata and removing commits. It works with any version control system that can export and import git fast-import streams, including git, hg, bzr, CVS, and RCS. It can also read Subversion dump files directly and can thus be used to script production of very high-quality conversions from Subversion to any supported DVCS."
Looks really promising. Haven’t tested it though.