how to access working tree from .git dir?

In messsing around with hooks, I've discovered that not all hooks are
run in the same environment.  In particular, the current working
directory in the post-receive hook (maybe others as well) is the GIT_DIR
(.git) directory, instead of the root of the working tree (as in
pre-commit).

This wouldn't be so bad, but it seems that `git rev-parse --show-cdup`
does not show anything valid if your current working directory is inside
the .git dir.  This creates a scenario where there is no consistent way
to access the root of the working tree from within a hook.  To make
things worse, the behavior changes subtly whether $GIT_DIR is defined or
not.

I've got a patch that changes the current working directory before
calling the post-receive hook, but that's more of a workaround than a
fix.  I'd like to be able to run `git rev-parse --show-cdup` from with
a .git directory and have it produce a valid result.

So:  I'm poking around around and trying to find the right answer but
some things look weird.  If anyone knows the quick-and-easy answer here
please let me know.  Otherwise, I'll continue to poke around some more.

Thanks,

-JE