Add workaround (2nd try)

https://rubyci.org/logs/rubyci.s3.amazonaws.com/gentoo/ruby-master/log/20190824T153002Z.fail.html.gz
```
/home/gentoo/chkbuild/tmp/build/20190824T153002Z/ruby/tool/lib/vcs.rb:577:in `export_changelog': cannot find the beginning revision of the branch (RuntimeError)
    from ./tool/make-snapshot:353:in `block in package'
    from ./tool/make-snapshot:351:in `chdir'
    from ./tool/make-snapshot:351:in `package'
    from ./tool/make-snapshot:523:in `block in <main>'
    from ./tool/make-snapshot:523:in `collect'
    from ./tool/make-snapshot:523:in `<main>'
```

tool/lib/vcs.rb

@@ -547,7 +547,7 @@ class VCS
         warn "retry trunk instead of master", uplevel: 0
         STDERR.puts "existing branches:"
         system(COMMAND, "branch", "-a", 1 => 2)
-        url = Branch.new("trunk")
+        url.to_str.replace("trunk")
         retry
       end
       raise