Color formatting fixed for wide strings (fixes issue #1594) (#1596)

* Use std::char_traits::length for ansi_color_escape::begin

-Fixes issue #1594 https://github.com/fmtlib/fmt/issues/1594

include/fmt/color.h

@@ -412,7 +412,7 @@ template <typename Char> struct ansi_color_escape {
 
   FMT_CONSTEXPR const Char* begin() const FMT_NOEXCEPT { return buffer; }
   FMT_CONSTEXPR const Char* end() const FMT_NOEXCEPT {
-    return buffer + std::strlen(buffer);
+    return buffer + std::char_traits<Char>::length(buffer);
   }
 
  private:

test/color-test.cc

@@ -50,6 +50,8 @@ TEST(ColorsTest, ColorsPrint) {
 TEST(ColorsTest, Format) {
   EXPECT_EQ(fmt::format(fg(fmt::rgb(255, 20, 30)), "rgb(255,20,30)"),
             "\x1b[38;2;255;020;030mrgb(255,20,30)\x1b[0m");
+  EXPECT_EQ(fmt::format(fg(fmt::rgb(255, 20, 30)), L"rgb(255,20,30) wide"),
+            L"\x1b[38;2;255;020;030mrgb(255,20,30) wide\x1b[0m");
   EXPECT_EQ(fmt::format(fg(fmt::color::blue), "blue"),
             "\x1b[38;2;000;000;255mblue\x1b[0m");
   EXPECT_EQ(