git-checkout: allow "checkout HEAD -- path"

Even though -- is redundant in this case, we should allow it to prevent
confusion.

Signed-off-by: Junio C Hamano <junkio@cox.net>

git-checkout.sh

diff --git a/git-checkout.sh b/git-checkout.sh
index 5613bfc..580a9e8 100755
--- a/git-checkout.sh
+++ b/git-checkout.sh
@@ -67,6 +67,10 @@
 			set x "$arg" "$@"
 			shift
 		fi
+		case "$1" in
+		--)
+			shift ;;
+		esac
 		break
 		;;
     esac