fix_bytes: loosen blacklist matching requirements On my Linux machine, paths seen by 2to3 include the build directory. We switch from an exact to substring match to allow 2to3 to work in more environments.

convert: only consider shamap revisions converted if they still exists Rollback or strip could leave a Mercurial repo with a shamap with revisions no longer in the repository. To ensure reliable conversions we now check that the commit actually exists and consider it non-existing if it doesn't exist.