Remove forward declaration of an enum

According to http://c-faq.com/null/machexamp.html, sizeof(char*) != sizeof(int*) on some platforms. Since an enum could be a char or int (or long or...), knowing the size of the enum thus is important to knowing the size of a pointer to an enum, so we cannot just forward declare an enum the way we can a struct. (Also, modern C++ compilers apparently define forward declarations of an enum to either be useless because the enum was defined, or require an explicit size specifier, or be a compilation error.) Helped-by: Jonathan Nieder <jrnieder@gmail.com> Signed-off-by: Elijah Newren <newren@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>