Add workaround for some CIs

https://rubyci.org/logs/rubyci.s3.amazonaws.com/debian8/ruby-master/log/20190824T093005Z.fail.html.gz
```
branches:
* trunk
  remotes/origin/trunk
```
and
```
fatal: Remote branch master not found in upstream origin
```

tool/lib/vcs.rb

@@ -540,9 +540,17 @@ class VCS
     end
 
     def export(revision, url, dir, keep_temp = false)
-      puts "branches:"; system(COMMAND, "branch", "-a") # for debug
       ret = system(COMMAND, "clone", "-s", (@srcdir || '.').to_s, "-b", url, dir)
       ret
+    rescue => e
+      if "master" == url.to_str && e.message == "Command failed with exit 128: git"
+        warn "retry trunk instead of master", uplevel: 0
+        STDERR.puts "existing branches:"
+        system(COMMAND, "branch", "-a", 1 => 2)
+        url = Branch.new("trunk")
+        retry
+      end
+      raise
     end
 
     def after_export(dir)