rev-parse --show-cdup in .git

I'm sure this was discussed somewhere, but I can't find it:

When called from within .git, git rev-parse --show-cdup returns nothing
rather than "..", not even an error code. Is this intended?

This is all the more disturbing since the cwd of hooks seems to be
GIT_DIR. Is that something one can rely upon? In that case one can
simply use ".." for cdup.

Michael

P.S.: The use case is a simple post-receive-hook that fast forwards a
checked out branch on receiving refs in refs/remotes/incoming/, but
needs to check the worktree against the index etc. 1.7.0 may be an
opportunity to encourage usage of this rather than restoring the old
behavior through options.