worktree: detect branch-name/detached and error conditions locally

worktree: detect branch-name/detached and error conditions locally

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

Once these operations are separate, git-worktree will no longer be able
to rely upon git-branch to determine the state of the worktree (branch
name or detached), or to check for error conditions, such as the
requested branch already checked out elsewhere, or an invalid reference.
Therefore, imbue git-worktree with the intelligence to determine a
branch name or detached state locally, and to perform error checking on
its own.

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