Make history infinite if set SAVE_HISTORY to negative

lib/irb/ext/save-history.rb

@@ -89,7 +89,7 @@ def load_history
     def save_history
       return unless self.class.const_defined?(:HISTORY)
       history = self.class::HISTORY
-      if num = IRB.conf[:SAVE_HISTORY] and (num = num.to_i) > 0
+      if num = IRB.conf[:SAVE_HISTORY] and (num = num.to_i) != 0
         if history_file = IRB.conf[:HISTORY_FILE]
           history_file = File.expand_path(history_file)
         end
@@ -110,7 +110,7 @@ def save_history
         open(history_file, "w:#{IRB.conf[:LC_MESSAGES].encoding}", 0600) do |f|
           hist = history.map{ |l| l.split("\n").join("\\\n") }
           begin
-            hist = hist.last(num) if hist.size > num
+            hist = hist.last(num) if hist.size > num and num > 0
           rescue RangeError # bignum too big to convert into `long'
             # Do nothing because the bignum should be treated as inifinity
           end

test/irb/test_history.rb

@@ -75,6 +75,29 @@ def test_history_save_bignum
       HISTORY_FILE
     end
 
+    def test_history_save_minus_as_infinity
+      result_output, result_history_file = launch_irb_with_irbrc_and_irb_history(<<~IRBRC, <<~IRB_HISTORY) do |stdin|
+        IRB.conf[:USE_READLINE] = true
+        IRB.conf[:SAVE_HISTORY] = -1 # infinity
+      IRBRC
+        1
+        2
+        3
+        4
+      IRB_HISTORY
+        stdin.write("5\nexit\n")
+      end
+
+      assert_equal(<<~HISTORY_FILE, result_history_file)
+        1
+        2
+        3
+        4
+        5
+        exit
+      HISTORY_FILE
+    end
+
     private
 
     def launch_irb_with_irbrc_and_irb_history(irbrc, irb_history)