dwim_ref: fix dangling symref warning

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>