checkout: add --ignore-skip-worktree-bits in sparse checkout mode

checkout: add --ignore-skip-worktree-bits in sparse checkout mode

"git checkout -- <paths>" is usually used to restore all modified
files in <paths>. In sparse checkout mode, this command is overloaded
with another meaning: to add back all files in <paths> that are
excluded by sparse patterns.

As the former makes more sense for day-to-day use. Switch it to the
default and the latter enabled with --ignore-skip-worktree-bits.

While at there, add info/sparse-checkout to gitrepository-layout.txt

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