Update cppfmt to current git.

vendor/Fmt/include/fmt/os.h

@@ -394,19 +394,15 @@ struct ostream_params {
 
 FMT_END_DETAIL_NAMESPACE
 
-constexpr detail::buffer_size buffer_size;
+// Added {} below to work around default constructor error known to
+// occur in Xcode versions 7.2.1 and 8.2.1.
+constexpr detail::buffer_size buffer_size{};
 
 /** A fast output stream which is not thread-safe. */
 class FMT_API ostream final : private detail::buffer<char> {
  private:
   file file_;
 
-  void flush() {
-    if (size() == 0) return;
-    file_.write(data(), size());
-    clear();
-  }
-
   void grow(size_t) override;
 
   ostream(cstring_view path, const detail::ostream_params& params)
@@ -426,6 +422,12 @@ class FMT_API ostream final : private detail::buffer<char> {
     delete[] data();
   }
 
+  void flush() {
+    if (size() == 0) return;
+    file_.write(data(), size());
+    clear();
+  }
+
   template <typename... T>
   friend ostream output_file(cstring_view path, T... params);