git-mergetool: properly handle "git mergetool -- filename"

Like many git commands, git-mergetool allows "--" to signal
the end of option processing.  This adds a missing "shift"
statement so that this is correctly handled.

Signed-off-by: David Aguilar <davvid@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-mergetool.sh

@@ -296,6 +296,7 @@ do
 	    esac
 	    ;;
 	--)
+	    shift
 	    break
 	    ;;
 	-*)