remote-hg: ensure shared repo is initialized

remote-hg: ensure shared repo is initialized

6796d49 (remote-hg: use a shared repository store) introduced a bug by
making the shared repository '.git/hg', which is already used before
that patch, so clones that happened before that patch, fail after that
patch, because there's no shared Mercurial repo.

So, instead of simply checking if the directory exists, let's always try
to create an empty shared repository to ensure it's there. This works
because we don't need the initial clone, if the repository is shared,
pulling from the child updates the parent's storage; it's exactly the
same as cloning, so we can simplify the shared repo setup this way while
at the same time fixing the problem.

Signed-off-by: Felipe Contreras <felipe.contreras@gmail.com>
Reviewed-by: Antoine Pelisse <apelisse@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>