Make 'new_branch' be the name of the new branch for both forced and
non-forced cases; and add boolean 'force_new_branch' to indicate forced
branch creation. This will simplify logic later on when git-worktree
handles branch creation locally rather than delegating it to
git-checkout as part of the worktree population phase.
Signed-off-by: Eric Sunshine <sunshine@sunshineco.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>