restore: add --worktree and --staged

'git checkout <tree-ish> <pathspec>' updates both index and
worktree. But updating the index when you want to restore worktree
files is non-intuitive. The index contains the data ready for the next
commit, and there's no indication that the user will want to commit
the restored versions.

'git restore' therefore by default only touches worktree. The user has
the option to update either the index with

    git restore --staged --source=<tree> <path>  (1)

or update both with

    git restore --staged --worktree --source=<tree> <path> (2)

PS. Orignally I wanted to make worktree update default and form (1)
would add index update while also updating the worktree, and the user
would need to do "--staged --no-worktree" to update index only. But it
looks really confusing that "--staged" option alone updates both. So
now form (2) is used for both, which reads much more obvious.

PPS. Yes form (1) overlaps with "git reset <rev> <path>". I don't know
if we can ever turn "git reset" back to "_always_ reset HEAD and
optionally do something else".

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
