scripts/feeds: ensure that --ff is enabled

scripts/feeds: ensure that --ff is enabled

Users may have merge.ff set to false, which will force merge commits to
be generated.  The intent here is to have a fast-forward merge when
possible, so let's make sure fast-forwards are enabled.

Signed-off-by: John Szakmeister <john@szakmeister.net>
Signed-off-by: Florian Fainelli <florian@openwrt.org>
git-svn-id: svn://svn.openwrt.org/openwrt/trunk@37884 3c298f89-4303-0410-b956-a3cf2f4a3e73