Simplify command method definition (#559)

* Remove unnecessary command argument generation code

Now that all the supported Ruby versions handle splat args and splat kwargs,
we don't need that args generation code anymore.

* Remove unused command definition code

If we look at `@EXTEND_COMMANDS`, all commands are defined in a file, which
means the `if load_file` branch is always executed. Therefore we can drop
the else branch of that condition.

* Avoid defining unnecessary command methods

There's no need to define another command method just to call `Command.execute`.

Author: st0012

lib/irb/extend-command.rb

@@ -246,7 +246,7 @@ def self.install_extend_commands
     #
     # The optional +load_file+ parameter will be required within the method
     # definition.
-    def self.def_extend_command(cmd_name, cmd_class, load_file = nil, *aliases)
+    def self.def_extend_command(cmd_name, cmd_class, load_file, *aliases)
       case cmd_class
       when Symbol
         cmd_class = cmd_class.id2name
@@ -255,33 +255,12 @@ def self.def_extend_command(cmd_name, cmd_class, load_file = nil, *aliases)
         cmd_class = cmd_class.name
       end
 
-      if load_file
-        line = __LINE__; eval %[
-          def #{cmd_name}(*opts, **kwargs, &b)
-            Kernel.require_relative "#{load_file}"
-            arity = ::IRB::ExtendCommand::#{cmd_class}.instance_method(:execute).arity
-            args = (1..(arity < 0 ? ~arity : arity)).map {|i| "arg" + i.to_s }
-            args << "*opts, **kwargs" if arity < 0
-            args << "&block"
-            args = args.join(", ")
-            line = __LINE__; eval %[
-              unless singleton_class.class_variable_defined?(:@@#{cmd_name}_)
-                singleton_class.class_variable_set(:@@#{cmd_name}_, true)
-                def self.#{cmd_name}_(\#{args})
-                  ::IRB::ExtendCommand::#{cmd_class}.execute(irb_context, \#{args})
-                end
-              end
-            ], nil, __FILE__, line
-            __send__ :#{cmd_name}_, *opts, **kwargs, &b
-          end
-        ], nil, __FILE__, line
-      else
-        line = __LINE__; eval %[
-          def #{cmd_name}(*opts, &b)
-            ::IRB::ExtendCommand::#{cmd_class}.execute(irb_context, *opts, &b)
-          end
-        ], nil, __FILE__, line
-      end
+      line = __LINE__; eval %[
+        def #{cmd_name}(*opts, **kwargs, &b)
+          Kernel.require_relative "#{load_file}"
+          ::IRB::ExtendCommand::#{cmd_class}.execute(irb_context, *opts, **kwargs, &b)
+        end
+      ], nil, __FILE__, line
 
       for ali, flag in aliases
         @ALIASES.push [ali, cmd_name, flag]