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

scripts/feeds

@@ -119,7 +119,7 @@ my %update_method = (
 	'src-git' => {
 		'init'          => "git clone --depth 1 '%s' '%s'",
 		'init_branch'   => "git clone --depth 1 --branch '%s' '%s' '%s'",
-		'update'	=> "git pull",
+		'update'	=> "git pull --ff",
 		'controldir'	=> ".git",
 		'revision'	=> "git show --abbrev-commit HEAD | head -n 1 | cut -d ' ' -f 2 | tr -d '\n'"},
 	'src-gitsvn' => {