scripts/feeds: shallow clone for specific commit update

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>