Fix formatter to skip trailing comma for single-line parameters

spec/compiler/formatter/formatter_spec.cr

@@ -1120,6 +1120,41 @@ describe Crystal::Formatter do
       )
       end
       CRYSTAL
+
+    assert_format <<-CRYSTAL, flags: %w[def_trailing_comma]
+      def foo(a)
+      end
+      CRYSTAL
+
+    assert_format <<-CRYSTAL, flags: %w[def_trailing_comma]
+      def foo(a, b)
+      end
+      CRYSTAL
+
+    assert_format <<-CRYSTAL, flags: %w[def_trailing_comma]
+      def foo(a, *args)
+      end
+      CRYSTAL
+
+    assert_format <<-CRYSTAL, flags: %w[def_trailing_comma]
+      def foo(a, *args, &block)
+      end
+      CRYSTAL
+
+    assert_format <<-CRYSTAL, flags: %w[def_trailing_comma]
+      def foo(a, **kwargs)
+      end
+      CRYSTAL
+
+    assert_format <<-CRYSTAL, flags: %w[def_trailing_comma]
+      def foo(a, **kwargs, &block)
+      end
+      CRYSTAL
+
+    assert_format <<-CRYSTAL, flags: %w[def_trailing_comma]
+      def foo(a, &block)
+      end
+      CRYSTAL
   end
 
   assert_format "1   +   2", "1 + 2"

src/compiler/crystal/tools/formatter.cr

@@ -1562,7 +1562,7 @@ module Crystal
 
       args.each_with_index do |arg, i|
         has_more = !last?(i, args) || double_splat || block_arg || yields || variadic
-        wrote_newline = format_def_arg(wrote_newline, has_more, true) do
+        wrote_newline = format_def_arg(wrote_newline, has_more, found_first_newline && !has_more) do
           if i == splat_index
             write_token :OP_STAR
             skip_space_or_newline
@@ -1577,7 +1577,7 @@ module Crystal
       end
 
       if double_splat
-        wrote_newline = format_def_arg(wrote_newline, block_arg || yields, true) do
+        wrote_newline = format_def_arg(wrote_newline, block_arg || yields, found_first_newline) do
           write_token :OP_STAR_STAR
           skip_space_or_newline