git-checkout: describe detached head correctly

When you have a file called HEAD in the work tree, the code to report
where the HEAD is at when "git checkout $commit^0" is done triggered
unnecessary ambiguity checking.

Explicitly mark the command line with "--" and make it clear that we are
talking about a revision.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-checkout.sh

@@ -169,7 +169,7 @@ detach_warn=
 describe_detached_head () {
 	test -n "$quiet" || {
 		printf >&2 "$1 "
-		GIT_PAGER= git log >&2 -1 --pretty=oneline --abbrev-commit "$2"
+		GIT_PAGER= git log >&2 -1 --pretty=oneline --abbrev-commit "$2" --
 	}
 }