clone: die when trying to clone missing local path

clone: die when trying to clone missing local path

Since 86ac751 (Allow cloning an empty repository,
2009-01-23), doing:

  git clone does-not-exist

has created does-not-exist as an empty repository. This was
an unintentional side effect of 86ac751. Even weirder,
doing:

  git clone does-not-exist new-dir

_does_ fail, making this "feature" (if you want to consider
it such) broken. Let's detect this situation and explicitly
die. It's almost certainly not what the user intended.

This patch also adds two tests. One for the missing path
case, and one to confirm that a similar case, cloning a
non-repository directory, fails.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>