git-ls-files -o no recurse?

git-ls-files -o reports _all_ the unknown files it finds in a work area. Subversion and probably other systems "simply ignore all the files and directories inside an unknown directory and just note the directory as unknown." (A quote from my porcelain's mailing list.)

I'd like to make this friendly behavior the default in my porcelain. Unfortunately, getting the same result from a porcelain requires implementing a non-trivial algorithm to discover common prefixes, worry about performance on large projects, etc.

Would it be hard to make git-ls-files optionally do this?

-- Darrin