git archive has confusing error messages

Hi,

Here's something I noticed back in 2012 or so, and stumbled upon again recently.

Start eg at the root of git's own repository:

$ cd xdiff/
$ git archive --format=tar HEAD .. | tar t
fatal: ..: '..' is outside repository at '/Users/matthias/prog/git'

In the best case, I would have hoped to get an archive of the whole
repository here.  At a minimum, I would hope for an error message that
was less confusing: the directory I was referring to is very much
inside the repository.

For comparison, `git -C .. archive --format=tar HEAD | tar t` has the
behaviour that I would expect in this scenario: a tarball of the whole
repository.

(I don't know enough about git to produce a patch to fix this at this
point, but I can try looking into fixing this.)

Thanks,
Matthias.