git-hg-synchronizer: use remote helper from Git's contrib/

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
git-hg-synchronizer: use bare repositories

There is no use to have a fetch being rejected because it wants to
update the master branch!

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>