Merge RubyGems 3.0.0.beta1.

* It drop to support < Ruby 2.2
  * Cleanup deprecated methods and classes.
  * Mark obsoleted methods to deprecate.
  * and other enhancements.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@63528 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

lib/rubygems/commands/update_command.rb

@@ -85,18 +85,27 @@ command to remove old versions.
   end
 
   def execute
-
     if options[:system] then
       update_rubygems
       return
     end
 
-    say "Updating installed gems"
-
     hig = highest_installed_gems
 
     gems_to_update = which_to_update hig, options[:args].uniq
 
+    if options[:explain]
+      say "Gems to update:"
+
+      gems_to_update.each do |(name, version)|
+        say "  #{name}-#{version}"
+      end
+
+      return
+    end
+
+    say "Updating installed gems"
+
     updated = update_gems gems_to_update
 
     updated_names = updated.map { |spec| spec.name }