worktree: populate via "git reset --hard" rather than "git checkout"

Now that git-worktree handles all functionality (--force, --detach,
-b/-B) previously delegated to git-checkout, actual population of the
new worktree can be accomplished more directly and lightweight with
"git reset --hard" in place of "git checkout".

Signed-off-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>