worktree: make setup of new HEAD distinct from worktree population

worktree: make setup of new HEAD distinct from worktree population

git-worktree currently conflates setting of HEAD in the new worktree and
initial worktree population into a single git-checkout invocation which
requires git-checkout to have special knowledge that it is operating on
a newly created worktree. The eventual goal is to rid git-checkout of
that overly-intimate knowledge.

Once these operations are separate, git-worktree will no longer be able
to delegate to git-branch the setting of the new worktree's HEAD to the
desired branch (or commit, if detached). Therefore, make git-worktree
itself responsible for setting up HEAD as either a symbolic reference,
if associated with a branch, or detached, if not.

Signed-off-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>