Disable printing the reset escape code when no style modifiers where applied. (#973)

include/fmt/color.h

@@ -438,20 +438,26 @@ template <
   typename S, typename Char = typename internal::char_t<S>::type>
 void vprint(std::FILE *f, const text_style &ts, const S &format,
             basic_format_args<typename buffer_context<Char>::type> args) {
+  bool has_style = false;
   if (ts.has_emphasis()) {
+    has_style = true;
     internal::fputs<Char>(
           internal::make_emphasis<Char>(ts.get_emphasis()), f);
   }
   if (ts.has_foreground()) {
+    has_style = true;
     internal::fputs<Char>(
           internal::make_foreground_color<Char>(ts.get_foreground()), f);
   }
   if (ts.has_background()) {
+    has_style = true;
     internal::fputs<Char>(
         internal::make_background_color<Char>(ts.get_background()), f);
   }
   vprint(f, format, args);
+  if (has_style) {
     internal::reset_color<Char>(f);
+  }
 }
 
 /**

test/format-impl-test.cc

@@ -232,4 +232,5 @@ TEST(ColorsTest, Colors) {
                "\x1b[1mbold error\x1b[0m");
   EXPECT_WRITE(stderr, fmt::print(stderr, fg(fmt::color::blue), "blue log"),
                  "\x1b[38;2;000;000;255mblue log\x1b[0m");
+  EXPECT_WRITE(stdout, fmt::print(fmt::text_style(), "hi"), "hi");
 }