worktree: simplify new branch (-b/-B) option checking
worktree: simplify new branch (-b/-B) option checking

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 <8d6e34f987851aa599257d3831a1af040886842f@sunshineco.com>
Signed-off-by: Junio C Hamano <a6723cc3f76163bf7adb636a73ac3b0ceb3e6b9b@pobox.com>

destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git