thin_check.cc: support --quiet in nested_output

thin-provisioning/thin_check.cc

@@ -51,7 +51,8 @@ namespace {
 		nested_output &operator <<(T const &t) {
 			if (beginning_of_line_) {
 				beginning_of_line_ = false;
-				indent();
+				if (enabled)
+					indent();
 			}
 
 			if (enabled)