Merge branch 'nd/checkout-m'

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