Hello again.
Grubba suggested one more change to perform during the filtering: Merges of single commits are converted to cherry-picks. Technically, this means that if a commit has two parents, and only one commit C is reachable from commit^2 but not commit^1, then the link to commit^2 is removed, and the commit message and author info of commit is replaced with that of C. I have only done this of commits created after the migration to git, and the complete list (25 commits) is on the Wiki page (http://pike-svn.lysator.liu.se/twiki/bin/view/Main/RepoFilter).
A new filtered repository which contains this additional fix is at git://pike-git.lysator.liu.se/pike-filtered-unmerged.git.
As before, questions and comments are welcome.