git filter-branch '--subdirectory-filter' preserving '--no-ff' merges.

(update2, 3/6/2013)

Well, in fact, the name is a bit misleading, because '--subdirectory-filter'
does not preserve merge commits created after fast-forward merge (with
'--no-ff' option). I think, this is because such commits are empty and do not
change any files in subdirectory (so, this is not a bug).

Anyway, i want to split subdirectory into separate project, preserving --no-ff
merge commits.