Rewrite instead of abandoning empty commits.

Fixes jj-vcs#2760


Given the tree:
```
A-B-C
 \
  B2
```
And the command `jj rebase -s B -d B2`

We were previously marking B as abandoned, despite the comment stating that we were marking it as being succeeded by B2. This resulted in a call to `rewrite(rewrites={}, abandoned={B})` instead of `rewrite(rewrites={B=>B2}, abandoned={})`, which then made the new parent of `C` into `A` instead of `B2`

lib/src/rewrite.rs

@@ -159,11 +159,11 @@ pub fn rebase_commit_with_options(
             EmptyBehaviour::AbandonAllEmpty => *parent.tree_id() == new_tree_id,
         };
         if should_abandon {
-            mut_repo.record_abandoned_commit(old_commit.id().clone());
             // Record old_commit as being succeeded by the parent for the purposes of
             // the rebase.
             // This ensures that when we stack commits, the second commit knows to
             // rebase on top of the parent commit, rather than the abandoned commit.
+            mut_repo.record_rewritten_commit(old_commit.id().clone(), parent.id().clone());
             return Ok(parent.clone());
         }
     }