git-checkout: disallow --detach on unborn branch

git-checkout: disallow --detach on unborn branch

abe199808c (git checkout -b: allow switching out of an unborn branch)
introduced a bug demonstrated by

  git checkout --orphan foo
  git checkout --detach
  git symbolic-ref HEAD

which gives 'refs/heads/(null)'.

This happens because we strbuf_addf(&branch_ref, "refs/heads/%s",
opts->new_branch) when opts->new_branch can be NULL for --detach.

Catch and forbid this case, adding a test to t2017 to catch it in
future.

Signed-off-by: Chris Webb <chris@arachsys.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>