worktree: make branch creation distinct from worktree population
git-worktree currently conflates branch creation, setting of HEAD in the
new worktree, and worktree population into a single sub-invocation of
git-checkout, which requires git-checkout to be specially aware that it
is operating in a newly-created worktree. The goal is to free
git-checkout of that special knowledge, and to do so, git-worktree will
eventually perform those operations separately. Thus, as a first step,
rather than piggybacking on git-checkout's -b/-B ability to create a new
branch at checkout time, make git-worktree responsible for branch
creation itself.
Signed-off-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>