branch: publish die_if_checked_out()

branch: publish die_if_checked_out()

git-worktree currently conflates new branch creation, setting of HEAD in
the new wortkree, and worktree population into a single sub-invocation
of git-checkout. However, these operations will eventually be separated,
and git-worktree itself will need to be able to detect if the branch is
already checked out elsewhere, rather than relying upon git-branch to
make this determination, so publish die_if_checked_out().

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