[Feature #20244] Issue a single Warning.warn call

Make the entire series of message lines a multiline string so that the
`Warning.warn` hook can receive them in a single call.

dir.c

@@ -1082,9 +1082,13 @@ chdir_alone_block_p(void)
         if (rb_thread_current() != chdir_lock.thread)
             rb_raise(rb_eRuntimeError, "conflicting chdir during another chdir block");
         if (!block_given) {
-            rb_warn("conflicting chdir during another chdir block");
             if (!NIL_P(chdir_lock.path)) {
-                rb_compile_warn(RSTRING_PTR(chdir_lock.path), chdir_lock.line, "here");
+                rb_warn("conflicting chdir during another chdir block\n"
+                        "%" PRIsVALUE ":%d: note: previous chdir was here",
+                        chdir_lock.path, chdir_lock.line);
+            }
+            else {
+                rb_warn("conflicting chdir during another chdir block");
             }
         }
     }

test/ruby/test_dir.rb

@@ -171,6 +171,26 @@ class TestDir < Test::Unit::TestCase
       42
     end
 
+    assert_separately(["-", @root], "#{<<~"begin;"}\n#{<<~'end;'}")
+    begin;
+      root = ARGV.shift
+
+      $dir_warnings = []
+
+      def Warning.warn(message)
+        $dir_warnings << message
+      end
+
+      line2 = line1 = __LINE__; Dir.chdir(root) do
+        line2 = __LINE__; Dir.chdir
+      end
+
+      message = $dir_warnings.shift
+      assert_include(message, "#{__FILE__}:#{line2}:")
+      assert_include(message, "#{__FILE__}:#{line1}:")
+      assert_empty($dir_warnings)
+    end;
+
     assert_equal(42, ret)
   ensure
     begin