dwim_ref: fix dangling symref warning

If we encounter a symref that is dangling, in most cases we will warn about it. The one exception is a dangling HEAD, as that indicates a branch yet to be born. However, the check in dwim_ref was not quite right. If we were fed something like "HEAD^0" we would try to resolve "HEAD", see that it is dangling, and then check whether the _original_ string we got was "HEAD" (which it wasn't in this case). And that makes no sense; the dangling thing we found was not "HEAD^0" but rather "HEAD". Fixing this squelches a scary warning from "submodule summary HEAD" (and consequently "git status" with status.submodulesummary set) in an empty repo, as the submodule script calls "git rev-parse -q --verify HEAD^0". Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>