Fix crash on TAB under ruby 2.7

When IRB from ruby 2.7 is loaded, it installs a Readline completion proc
that assumes IRB is running (and crashes otherwise). Workaround this by
clearing the Readline completion when using it directly.

Fixes #654

lib/byebug/interfaces/local_interface.rb

@@ -21,7 +21,7 @@ def initialize
     # @param prompt Prompt to be displayed.
     #
     def readline(prompt)
-      with_repl_like_sigint { Readline.readline(prompt) || EOF_ALIAS }
+      with_repl_like_sigint { without_irb_completion { Readline.readline(prompt) || EOF_ALIAS } }
     end
 
     #
@@ -40,5 +40,14 @@ def with_repl_like_sigint
     ensure
       trap("INT", orig_handler)
     end
+
+    # Disables the Readline completion proc installed by IRB on ruby2.7
+    def without_irb_completion
+      orig_completion = Readline.completion_proc
+      Readline.completion_proc = nil
+      yield
+    ensure
+      Readline.completion_proc = orig_completion
+    end
   end
 end