Improve Array#concat performance if only one argument is given

* array.c (rb_ary_concat_multi): concatenate the array without generating
    temporary Array object if only one argument is given.
    This is very similar with r58886.

    Array#concat will be faster around 19%.
    [Fix GH-1634]

### Before
        Array#concat      2.187M (± 3.5%) i/s -     10.926M in   5.002829s

### After
        Array#concat      2.598M (± 1.8%) i/s -     13.008M in   5.008201s

### Test code
require 'benchmark/ips'

Benchmark.ips do |x|
  x.report "Array#concat" do |i|
    other = [4]
    i.times { [1, 2, 3].concat(other) }
  end
end

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@58909 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

array.c

@@ -3679,7 +3679,10 @@ rb_ary_concat_multi(int argc, VALUE *argv, VALUE ary)
 {
     rb_ary_modify_check(ary);
 
-    if (argc > 0) {
+    if (argc == 1) {
+	rb_ary_concat(ary, argv[0]);
+    }
+    else if (argc > 1) {
 	int i;
 	VALUE args = rb_ary_tmp_new(argc);
 	for (i = 0; i < argc; i++) {