Merge branch 'nd/checkout-m'

"git checkout -m <other>" was about carrying the differences between HEAD and the working-tree files forward while checking out another branch, and ignored the differences between HEAD and the index. The command has been taught to abort when the index and the HEAD are different. * nd/checkout-m: checkout: prevent losing staged changes with --merge read-tree: add --quiet unpack-trees: rename "gently" flag to "quiet" unpack-trees: keep gently check inside add_rejected_path