opkg: switch to git repository (#14655)

Upstream development now takes place in a git repository which was clone
from the old svn repository. Switch the source location to the git tree
using the commit that corresponds to the old svn r618 revision.

Signed-off-by: Jo-Philipp Wich <jow@openwrt.org>
git-svn-id: svn://svn.openwrt.org/openwrt/trunk@39153 3c298f89-4303-0410-b956-a3cf2f4a3e73