update: clone from a cache with –no-checkout

When a project is cloned from a cache, west checks out the manifest revision right afterwards. Until now, the files were first written at the HEAD of the cache by ‘git clone’ and ‘git checkout –detach’, and right afterwards again at the manifest revision. This matters for big projects like zephyr (~60k files).

Clone with –no-checkout and detach HEAD with ‘git update-ref –no-deref’ instead, so the files are written only once, at the manifest revision.