Remote: incorporate callbacks into the struct

Let a user with a Remote object use it, same as it's wired up for clone.

remote.go

@@ -55,6 +55,8 @@ type RemoteCallbacks struct {
 type Remote struct {
 	ptr              *C.git_remote
 	repo             *Repository
+
+	Callbacks RemoteCallbacks
 }
 
 func newRemote(cremote *C.git_remote, repo *Repository) *Remote {
@@ -63,6 +65,9 @@ func newRemote(cremote *C.git_remote, repo *Repository) *Remote {
 		repo: repo,
 	}
 
+	var callbacks C.git_remote_callbacks
+	populateRemoteCallbacks(&callbacks, &remote.Callbacks)
+
 	runtime.SetFinalizer(remote, (*Remote).Free)
 	return remote
 }