scripts/feeds: shallow clone submodules

When a feed has submodules, all its submodules are fully cloned whereas
the feed itself is shallowed. Let's be consistent and perform shallow clones
as well for the submodules.

Signed-off-by: Cedric CHEDALEUX <cedric.chedaleux@orange.com>
Link: https://github.com/openwrt/openwrt/pull/18003
Signed-off-by: Robert Marko <robimarko@gmail.com>

scripts/feeds

@@ -162,7 +162,7 @@ my %update_method = (
 		'update_rebase'	=> "git pull --rebase=merges",
 		'update_stash'	=> "git pull --rebase=merges --autostash",
 		'update_force'	=> "git pull --ff-only || (git reset --hard HEAD; git pull --ff-only; exit 1)",
-		'post_update'	=> "git submodule update --init --recursive",
+		'post_update'	=> "git submodule update --init --recursive --depth 1",
 		'controldir'	=> ".git",
 		'revision'	=> "git rev-parse HEAD | tr -d '\n'"},
 	'src-git-full' => {