lib: make DescendantRebaser-related APIs private.

Finally, there are no test uses of these APIs. `DescendantRebaser` is made
`pub(crate)`, since it is used by `MutRepo`. Other functions are made private.

lib/src/repo.rs

@@ -885,7 +885,7 @@ impl MutableRepo {
     /// Creates a `DescendantRebaser` to rebase descendants of the recorded
     /// rewritten and abandoned commits.
     // TODO(ilyagr): Inline this. It's only used in tests.
-    pub fn create_descendant_rebaser<'settings, 'repo>(
+    fn create_descendant_rebaser<'settings, 'repo>(
         &'repo mut self,
         settings: &'settings UserSettings,
     ) -> DescendantRebaser<'settings, 'repo> {

lib/src/rewrite.rs

@@ -242,7 +242,7 @@ pub struct RebaseOptions {
 }
 
 /// Rebases descendants of a commit onto a new commit (or several).
-pub struct DescendantRebaser<'settings, 'repo> {
+pub(crate) struct DescendantRebaser<'settings, 'repo> {
     settings: &'settings UserSettings,
     mut_repo: &'repo mut MutableRepo,
     // The commit identified by the key has been replaced by all the ones in the value, typically
@@ -521,7 +521,7 @@ impl<'settings, 'repo> DescendantRebaser<'settings, 'repo> {
 
     // TODO: Perhaps change the interface since it's not just about rebasing
     // commits.
-    pub fn rebase_next(&mut self) -> Result<Option<RebasedDescendant>, TreeMergeError> {
+    fn rebase_next(&mut self) -> Result<Option<RebasedDescendant>, TreeMergeError> {
         while let Some(old_commit) = self.to_visit.pop() {
             let old_commit_id = old_commit.id().clone();
             if let Some(new_parent_ids) = self.parent_mapping.get(&old_commit_id).cloned() {