The plan is to publish die_if_checked_out() so that callers other than
git-checkout can take advantage of it, however, those callers won't have
access to git-checkout's "struct branch_info". Therefore, change it to
accept the full name of the branch as a simple string instead.
While here, also give the argument a more meaningful name ("branch"
instead of "new").
Signed-off-by: Eric Sunshine <sunshine@sunshineco.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>