remote-hg: add shared repo upgrade

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.

It's trivial to upgrade to the new organization by copying the Mercurial
repo from one of the remotes (e.g. 'origin'), so let's do so.

Reported-by: Joern Hees <dev@joernhees.de>
Signed-off-by: Felipe Contreras <felipe.contreras@gmail.com>