Switch to StdioInputMethod when TERM is 'dumb'

This works around Reline's misbehavior on low-capability terminals,
such as when IRB is launched inside Emacs (ruby/reline#616).

It should also resolve ruby#68 and resolve ruby#113 which were filed out of
similar need.

lib/irb.rb

@@ -312,7 +312,9 @@
 # ### Input Method
 #
 # The IRB input method determines how command input is to be read; by default,
-# the input method for a session is IRB::RelineInputMethod.
+# the input method for a session is IRB::RelineInputMethod. Unless the
+# value of the TERM environment variable is 'dumb', in which case the
+# most simplistic input method is used.
 #
 # You can set the input method by:
 #
@@ -330,7 +332,8 @@
 #         IRB::ReadlineInputMethod.
 #     *   `--nosingleline` or `--multiline` sets the input method to
 #         IRB::RelineInputMethod.
-#
+#     *   `--nosingleline` together with `--nomultiline` sets the
+#         input to IRB::StdioInputMethod.
 #
 #
 # Method `conf.use_multiline?` and its synonym `conf.use_reline` return:

lib/irb/context.rb

@@ -85,7 +85,7 @@ def initialize(irb, workspace = nil, input_method = nil)
         @io = nil
         case use_multiline?
         when nil
-          if STDIN.tty? && IRB.conf[:PROMPT_MODE] != :INF_RUBY && !use_singleline?
+          if term_interactive? && IRB.conf[:PROMPT_MODE] != :INF_RUBY && !use_singleline?
             # Both of multiline mode and singleline mode aren't specified.
             @io = RelineInputMethod.new(build_completor)
           else
@@ -99,7 +99,7 @@ def initialize(irb, workspace = nil, input_method = nil)
         unless @io
           case use_singleline?
           when nil
-            if (defined?(ReadlineInputMethod) && STDIN.tty? &&
+            if (defined?(ReadlineInputMethod) && term_interactive? &&
                 IRB.conf[:PROMPT_MODE] != :INF_RUBY)
               @io = ReadlineInputMethod.new
             else
@@ -151,6 +151,10 @@ def initialize(irb, workspace = nil, input_method = nil)
       @command_aliases = @user_aliases.merge(KEYWORD_ALIASES)
     end
 
+    private def term_interactive?
+      STDIN.tty? && ENV['TERM'] != 'dumb'
+    end
+
     # because all input will eventually be evaluated as Ruby code,
     # command names that conflict with Ruby keywords need special workaround
     # we can remove them once we implemented a better command system for IRB