cmd/releasebot: remove support for parallel releases

The output would end up mixed to the point of being useless. releasebot
now supports parallelism at the terminal tab level.

Change-Id: I291954a88884452e71d3e3d0416301453aec8374
Reviewed-on: https://go-review.googlesource.com/c/154340
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>

cmd/releasebot/main.go

@@ -64,7 +64,7 @@ func main() {
 	security := flag.Bool("security", false, "cut a security release from the internal Gerrit")
 	flag.Usage = usage
 	flag.Parse()
-	if *modeFlag == "" || !releaseModes[*modeFlag] || flag.NArg() == 0 {
+	if *modeFlag == "" || !releaseModes[*modeFlag] || flag.NArg() != 1 {
 		usage()
 	}
 
@@ -77,8 +77,8 @@ func main() {
 	loadGithubAuth()
 	loadGCSAuth()
 
-	var wg sync.WaitGroup
-	for _, release := range flag.Args() {
+	release := flag.Arg(0)
+
 	if strings.Contains(release, "beta") || strings.Contains(release, "rc") {
 		if *security {
 			log.Printf("error: only minor releases are supported in security mode")
@@ -90,12 +90,8 @@ func main() {
 			BetaRelease: strings.Contains(release, "beta"),
 			RCRelease:   strings.Contains(release, "rc"),
 		}
-			wg.Add(1)
-			go func() {
-				defer wg.Done()
 		w.doRelease()
-			}()
-			continue
+		return
 	}
 
 	errFoundMilestone := errors.New("found milestone")
@@ -112,24 +108,18 @@ func main() {
 				Version:       release,
 				Security:      *security,
 			}
-				wg.Add(1)
-				go func() {
-					defer wg.Done()
 			w.doRelease()
-				}()
 			return errFoundMilestone
 		}
 		return nil
 	})
 	if err != nil && err != errFoundMilestone {
-			log.Printf("error looking for release %s: %v", release, err)
+		log.Fatalf("error looking for release %s: %v", release, err)
 	}
 	if err == nil {
-			log.Printf("cannot find release %s", release)
+		log.Fatalf("cannot find release %s", release)
 	}
 }
-	wg.Wait()
-}
 
 func nextMilestone(m *maintner.GitHubMilestone) (*maintner.GitHubMilestone, error) {
 	titleParts := strings.Split(m.Title, ".")