remotes-hg: bugfix for fetching non local remotes

6796d49 introduced a bug by making shared_path == ".git/hg' which
will most likely exist already, causing a new remote never to be
cloned and subsequently causing hg.share to fail with error msg:
"mercurial.error.RepoError: repository .git/hg not found"

Changing gitdir to dirname causes shared_path ==
.git/hg/<remote_name>/hg. The call to hg.share with local_path ==
.git/hg/<remote_name>/clone works again.

Signed-off-by: Joern Hees <dev@joernhees.de>