bisect: fix "reset" when branch is checked out elsewhere

bisect: fix "reset" when branch is checked out elsewhere

Since 1d0fa89 (checkout: add --ignore-other-wortrees, 2015-01-03) we
have a safety valve in checkout/switch to prevent the same branch from
being checked out simultaneously in multiple worktrees.

If a branch is bisected in a worktree while also being checked out in
another worktree; when the bisection is finished, checking out the
branch back in the current worktree may fail.

Let's teach bisect to use the "--ignore-other-worktrees" flag.

Signed-off-by: Rubén Justo <rjusto@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>