pub type git_diff_file_cb = Option<extern "C" fn(_: *const git_diff_delta, _: f32, _: *mut c_void) -> c_int>;
enum git_diff_file_cb { None, Some(extern "C" fn(_: *const git_diff_delta, _: f32, _: *mut c_void) -> i32), }
No value.
Some value of type T.
T