repo: make RewriteType private

Looks like I forgot this in some recent refactoring.

I don't really see any harm in making the type public later. I might
want to make `rebase_descendants()` not clear `parent_mapping` and
instead provide a way of accessing it afterwards (removing the need
for the `_return_map()` flavors).  We'll see if that ends up
happening. For now it can be private anyway.

lib/src/repo.rs

@@ -762,7 +762,7 @@ impl RepoLoader {
 }
 
 #[derive(Clone, Copy, Debug, PartialEq, Eq)]
-pub(crate) enum RewriteType {
+enum RewriteType {
     Rewritten,
     Divergent,
     Abandoned,