remote-hg: fix paths with spaces

git-fast-import expects paths to be C-style quoted, so let's do that the
same way git-remote-bzr does.

Signed-off-by: Antoine Pelisse <apelisse@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
Improved-by: Richard Hansen <rhansen@bbn.com>
Signed-off-by: Felipe Contreras <felipe.contreras@gmail.com>
remote-hg: add tests for special filenames

So that we check that UTF-8 and spaces work fine.

Signed-off-by: Felipe Contreras <felipe.contreras@gmail.com>