rewrite: exclude already rewritten commits from set to rebase

We currently include the commits in `parent_mapping` and `abandoned`
in the set of commits to visit when rebasing descendants. The reason
was that we used to update branches and working copies when we visited
these commits. Since we started updating refs after rebasing all
commits, there's no need to even visit these commits.

lib/src/rewrite.rs

@@ -301,7 +301,9 @@ impl<'settings, 'repo> DescendantRebaser<'settings, 'repo> {
             RevsetExpression::commits(mut_repo.parent_mapping.keys().cloned().collect()).union(
                 &RevsetExpression::commits(mut_repo.abandoned.iter().cloned().collect()),
             );
-        let to_visit_expression = old_commits_expression.descendants();
+        let to_visit_expression = old_commits_expression
+            .descendants()
+            .minus(&old_commits_expression);
         let to_visit_revset = to_visit_expression.evaluate_programmatic(mut_repo).unwrap();
         let to_visit: Vec<_> = to_visit_revset.iter().commits(store).try_collect().unwrap();
         drop(to_visit_revset);
@@ -439,12 +441,7 @@ impl<'settings, 'repo> DescendantRebaser<'settings, 'repo> {
 
     fn rebase_one(&mut self, old_commit: Commit) -> Result<(), TreeMergeError> {
         let old_commit_id = old_commit.id().clone();
-        if self.mut_repo.parent_mapping.contains_key(&old_commit_id) {
-            // This is a commit that had already been rebased before `self` was created
-            // (i.e. it's part of the input for this rebase). We don't need
-            // to rebase it.
-            return Ok(());
-        }
+        assert!(!self.mut_repo.parent_mapping.contains_key(&old_commit_id));
         let old_parent_ids = old_commit.parent_ids();
         let new_parent_ids = self.mut_repo.new_parents(old_parent_ids);
         if new_parent_ids == old_parent_ids {