mtimer: start the search for a dir time with unix(0)

if the dir is newer than the files then we still want to force it to a
consistent value of the files

most notably this should prevent a whole host of dirs from having an
mtime that is the package unpack time, which is obviously changing
between builds

mtimer/main.go

@@ -107,7 +107,7 @@ func updateDir(dir DirInfo) {
 		log.Fatal(err)
 	}
 
-	latest := dir.info.ModTime()
+	latest := time.Unix(0, 0)
 	for _, entry := range entries {
 		if entry.Type()&os.ModeSymlink != 0 {
 			continue