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