cli rebase: fix an assumption in a comment and add a TODO

It In Git, a commit's direct parent is allowed to also be an indirect ancestor
at the same time. `jj` currently tries to prevent this situation, but does
allow it. The correctness of `rebase -r A -d descendant_of_A` currently depends
on this jj-specific behavior; we should change that.

Cc jj-vcs#2600

This is an independent problem from the one the parent commit describes,
though it affects the same piece of code.

cli/src/commands/rebase.rs

@@ -369,14 +369,18 @@ fn rebase_revision(
     let num_rebased_descendants = rebased_commit_ids.len();
 
     // We now update `new_parents` to account for the rebase of all of
-    // `old_commit`'s descendants. Even if some of the original `new_parents` were
-    // descendants of `old_commit`, this will no longer be the case after the
-    // update.
+    // `old_commit`'s descendants. Even if some of the original `new_parents`
+    // were descendants of `old_commit`, this will no longer be the case after
+    // the update.
     //
     // To make the update simpler, we assume that each commit was rewritten only
     // once; we don't have a situation where both `(A,B)` and `(B,C)` are in
-    // `rebased_commit_ids`. This assumption relies on the fact that a descendant of
-    // a child of `old_commit` cannot also be a direct child of `old_commit`.
+    // `rebased_commit_ids`.
+    // TODO(ilyagr): This assumption relies on the fact that, after
+    // `rebase_descendants`, a child of `old_commit` cannot also be a direct
+    // child of `old_commit`. This will likely change, see
+    // https://github.com/martinvonz/jj/issues/2600. So, the code needs to be
+    // updated before that happens.
     let new_parents: Vec<_> = new_parents
         .iter()
         .map(|new_parent| {