rewrite: remove return value from rebase_next()

`rebase_next()` returns an `Option<RebasedDescendant>`, but the only
way we use it is to decide whether to terminate the loop over
`to_visit`. Let's simplify by making the caller iterate over
`to_visit` instead.

lib/src/rewrite.rs

@@ -539,8 +539,7 @@ impl<'settings, 'repo> DescendantRebaser<'settings, 'repo> {
         Ok(())
     }
 
-    fn rebase_next(&mut self) -> Result<Option<RebasedDescendant>, TreeMergeError> {
-        while let Some(old_commit) = self.to_visit.pop() {
+    fn rebase_one(&mut self, old_commit: Commit) -> Result<(), TreeMergeError> {
         let old_commit_id = old_commit.id().clone();
         if let Some(new_parent_ids) = self.parent_mapping.get(&old_commit_id).cloned() {
             // This is a commit that had already been rebased before `self` was created
@@ -548,13 +547,13 @@ impl<'settings, 'repo> DescendantRebaser<'settings, 'repo> {
             // to rebase it, but we still want to update branches pointing
             // to the old commit.
             self.update_references(old_commit_id, new_parent_ids, false)?;
-                continue;
+            return Ok(());
         }
         if let Some(divergent_ids) = self.divergent.get(&old_commit_id).cloned() {
             // Leave divergent commits in place. Don't update `parent_mapping` since we
             // don't want to rebase descendants either.
             self.update_references(old_commit_id, divergent_ids, false)?;
-                continue;
+            return Ok(());
         }
         let old_parent_ids = old_commit.parent_ids();
         let new_parent_ids = self.new_parents(old_parent_ids);
@@ -563,10 +562,10 @@ impl<'settings, 'repo> DescendantRebaser<'settings, 'repo> {
             self.parent_mapping
                 .insert(old_commit_id.clone(), new_parent_ids.clone());
             self.update_references(old_commit_id, new_parent_ids, true)?;
-                continue;
+            return Ok(());
         } else if new_parent_ids == old_parent_ids {
             // The commit is already in place.
-                continue;
+            return Ok(());
         }
 
         // Don't create commit where one parent is an ancestor of another.
@@ -604,16 +603,13 @@ impl<'settings, 'repo> DescendantRebaser<'settings, 'repo> {
             vec![new_commit.id().clone()],
             abandoned_old_commit,
         )?;
-            return Ok(Some(RebasedDescendant {
-                old_commit,
-                new_commit,
-            }));
-        }
-        Ok(None)
+        Ok(())
     }
 
     pub fn rebase_all(&mut self) -> Result<(), TreeMergeError> {
-        while self.rebase_next()?.is_some() {}
+        while let Some(old_commit) = self.to_visit.pop() {
+            self.rebase_one(old_commit)?;
+        }
         let mut view = self.mut_repo.view().store_view().clone();
         for commit_id in &self.heads_to_remove {
             view.head_ids.remove(commit_id);
@@ -627,9 +623,3 @@ impl<'settings, 'repo> DescendantRebaser<'settings, 'repo> {
         Ok(())
     }
 }
-
-#[derive(Debug, PartialEq, Eq, Clone)]
-pub struct RebasedDescendant {
-    pub old_commit: Commit,
-    pub new_commit: Commit,
-}