Move require out of repeated execution path

SHOW_DOC_DIALOG will be called repeatedly whenever the corresponding key
is pressed, but we only need to require rdoc once. So ideally the
require can be put outside of the proc.

And because when rdoc is not available the entire proc will be
nonfunctional, we can stop registering the SHOW_DOC_DIALOG if we failed
to require rdoc.

Author: st0012

lib/irb/input-method.rb

@@ -296,8 +296,13 @@ def initialize
         end
       Reline.dig_perfect_match_proc = IRB::InputCompletor::PerfectMatchedProc
       Reline.autocompletion = IRB.conf[:USE_AUTOCOMPLETE]
+
       if IRB.conf[:USE_AUTOCOMPLETE]
-        Reline.add_dialog_proc(:show_doc, SHOW_DOC_DIALOG, Reline::DEFAULT_DIALOG_CONTEXT)
+        begin
+          require 'rdoc'
+          Reline.add_dialog_proc(:show_doc, SHOW_DOC_DIALOG, Reline::DEFAULT_DIALOG_CONTEXT)
+        rescue LoadError
+        end
       end
     end
 
@@ -321,11 +326,6 @@ def auto_indent(&block)
         [195, 164], # The "ä" that appears when Alt+d is pressed on xterm.
         [226, 136, 130] # The "∂" that appears when Alt+d in pressed on iTerm2.
       ]
-      begin
-        require 'rdoc'
-      rescue LoadError
-        return nil
-      end
 
       if just_cursor_moving and completion_journey_data.nil?
         return nil