scripts/feeds: shallow clone for specific commit update

When a feed is referenced with a specific commit (i.e. <git_url>^<sha1>),
a full clone was performed and a branch was created from the sha1
and named with the sha1. Other git clones operations are shallowed.

As Git does not support clone at a specific commit, let's first perform
a shallow clone to latest commit, then fetch the relevant commit and
finally checkout it (no more 'pseudo' branch).

It saves bandwith and significantly speeds up the feed update process.

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

@@ -157,7 +157,7 @@ my %update_method = (
 	'src-git' => {
 		'init'          => "git clone --depth 1 '%s' '%s'",
 		'init_branch'   => "git clone --depth 1 --branch '%s' '%s' '%s'",
-		'init_commit'   => "git clone '%s' '%s' && cd '%s' && git checkout -b '%s' '%s' && cd -",
+		'init_commit'   => "git clone --depth 1 '%s' '%s' && cd '%s' && git fetch --depth=1 origin '%s' && git -c advice.detachedHead=false checkout '%s' && cd -",
 		'update'	=> "git pull --ff-only",
 		'update_rebase'	=> "git pull --rebase=merges",
 		'update_stash'	=> "git pull --rebase=merges --autostash",