remote-hg: fix commit messages

git fast-import expects an extra newline after the commit message data,
but we are adding it only on hg-git compat mode, which is why the
bidirectionality tests pass.

We should add it unconditionally.

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