builder: don't use logger date/time

Logger default date/time are prefixes and don't play well when
printing multiple lines.  What usually matters is the amount of time
elapsed since the start, so print that instead (when each new entry
that is not the first is starting).

builder/stopwatch.go

@@ -21,12 +21,20 @@ type stopWatchEntry struct {
 }
 
 func (sw *stopWatch) Start(name string) {
-	logger := log.New(sw.w, "", log.Ldate|log.Ltime)
+	logger := log.New(sw.w, "", 0)
 	if sw.w != nil {
 		if len(sw.entries) > 0 {
 			logger.Println()
+
+			var d time.Duration
+			for _, e := range sw.entries {
+				d += e.d
+			}
+			logger.Printf("=> %s (%s elapsed since start)\n", name,
+				d.Truncate(time.Millisecond))
+		} else {
+			logger.Printf("=> %s\n", name)
 		}
-		logger.Printf("=> %s\n", name)
 	}
 	sw.entries = append(sw.entries, stopWatchEntry{name: name})
 	sw.t = time.Now()
@@ -47,7 +55,7 @@ func (sw *stopWatch) Stop() {
 }
 
 func (sw *stopWatch) WriteSummary(w io.Writer) {
-	logger := log.New(w, "", log.Ldate|log.Ltime)
+	logger := log.New(w, "", 0)
 	if len(sw.entries) == 0 {
 		return
 	}