rbinstall.rb: install just one gemspec [ci skip]

* tool/rbinstall.rb (gem): install gemspec match with the
  directory only.  some gems, (e.g., json) have multiple gemspec
  files.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@56070 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

tool/rbinstall.rb

@@ -761,6 +761,7 @@ install?(:ext, :comm, :gem) do
       Gem::Specification.load(File.basename(path))
     }
     next unless spec.platform == Gem::Platform::RUBY
+    next unless spec.full_name == path[srcdir.size..-1][/\A\/gems\/([^\/]+)/, 1]
     spec.extension_dir = "#{extensions_dir}/#{spec.full_name}"
     if File.directory?(ext = "#{gem_ext_dir}/#{spec.full_name}")
       spec.extensions[0] ||= "-"