rev-parse's show-{cdup,prefix} inconsistent when pwd symlink

`git rev-parse --show-prefix` gives the symlinked directory, not pwd:

``` $ git init Initialized empty Git repository in [~]/git-test/.git/ $ mkdir realdir $ ln -s realdir symdir $ cd symdir $ pwd [~]/misc/git-test/symdir $ git rev-parse --show-prefix realdir/ ```

That might arguably be desirable, (though not in my prompt) but I don't think this is:

``` $ cd .. $ mkdir realdir/2 $ ln -s realdir/2 symdir2 $ cd symdir2 $ pwd [~]/misc/git-test/symdir2 $ git rev-parse --show-cdup ../../ ```

that is, `--show-cdup` gives a 'cd up' that is incorrect for where we actually are.

Apologies if I've just missed an option,