restore: replace --force with --ignore-unmerged

restore: replace --force with --ignore-unmerged

Use a more specific option name to express its purpose. --force may come
back as an alias of --ignore-unmerged and possibly more. But since this
is a destructive operation, I don't see why we need to "force" anything
more. We already don't hold back.

When 'checkout --force' or 'restore --ignore-unmerged' is used, we may
also print warnings about unmerged entries being ignore. Since this is
not exactly warning (people tell us to do so), more informational, let
it be suppressed if --quiet is given. This is a behavior change for
git-checkout.

PS. The diff looks a bit iffy since --force is moved to
add_common_switch_branch_options() (i.e. for switching). But
git-checkout is also doing switching and inherits this --force.

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