Fix rebase -r of a parent of a merge commit

Previously, using `rebase -r` on the parent of a merge commit
turned it into a non-merge commit. In other words, starting
with

```
    o   d
    |\
    o | c
    o | b
    | o a
    |/  
    o 
```

and doing `rebase -r c -d a` resulted in

```
    o d
    o b
    | o c
    | o a
    |/  
    o 
```

where `d` is no longer a merge commit.

For reference, here's a complete test that passed before this commit (but should NOT pass; see the diff for a test that should pass):

```
#[test]
fn test_rebase_single_revision_merge_parent() {
    let test_env = TestEnvironment::default();
    test_env.jj_cmd_success(test_env.env_root(), &["init", "repo", "--git"]);
    let repo_path = test_env.env_root().join("repo");

    create_commit(&test_env, &repo_path, "a", &[]);
    create_commit(&test_env, &repo_path, "b", &[]);
    create_commit(&test_env, &repo_path, "c", &["b"]);
    create_commit(&test_env, &repo_path, "d", &["a", "c"]);
    // Test the setup
    insta::assert_snapshot!(get_log_output(&test_env, &repo_path), @r###"
    @
    o   d
    |\
    o | c
    o | b
    | o a
    |/
    o
    "###);

    let stdout = test_env.jj_cmd_success(&repo_path, &["rebase", "-r", "c", "-d", "a"]);
    insta::assert_snapshot!(stdout, @r###"
    Also rebased 2 descendant commits onto parent of rebased commit
    Working copy now at: 3e176b54d680 (no description set)
    Added 0 files, modified 0 files, removed 2 files
    "###);
    insta::assert_snapshot!(get_log_output(&test_env, &repo_path), @r###"
    @
    o d
    | o c
    o | b
    | o a
    |/
    o
    "###);
}
```

src/commands.rs

@@ -4130,6 +4130,7 @@ fn rebase_revision(
     let children_expression = RevsetExpression::commit(old_commit.id().clone()).children();
     let mut num_rebased_descendants = 0;
     let store = workspace_command.repo.store();
+
     for child_commit in children_expression
         .evaluate(
             workspace_command.repo().as_repo_ref(),
@@ -4139,11 +4140,45 @@ fn rebase_revision(
         .iter()
         .commits(store)
     {
+        let child_commit = child_commit?;
+        let new_parent_ids: Vec<CommitId> = child_commit
+            .parents()
+            .iter()
+            .flat_map(|c| {
+                if c == &old_commit {
+                    old_commit
+                        .parents()
+                        .iter()
+                        .map(|c| c.id().clone())
+                        .collect()
+                } else {
+                    [c.id().clone()].to_vec()
+                }
+            })
+            .collect();
+
+        // Some of `new_parents` may be ancestors of other elements of `new_parents`
+        // as in `test_rebase_single_revision`.
+        let new_child_parents: Result<Vec<Commit>, BackendError> = RevsetExpression::Difference(
+            RevsetExpression::commits(new_parent_ids.clone()),
+            RevsetExpression::commits(new_parent_ids.clone())
+                .parents()
+                .ancestors(),
+        )
+        .evaluate(
+            workspace_command.repo().as_repo_ref(),
+            Some(&workspace_command.workspace_id()),
+        )
+        .unwrap()
+        .iter()
+        .commits(store)
+        .collect();
+
         rebase_commit(
             ui.settings(),
             tx.mut_repo(),
-            &child_commit?,
-            &old_commit.parents(),
+            &child_commit,
+            &new_child_parents?,
         );
         num_rebased_descendants += 1;
     }

tests/test_rebase_command.rs

@@ -186,30 +186,34 @@ fn test_rebase_single_revision() {
     o 
     "###);
 
-    // Descendants of the rebased commit should be rebased onto parents. First we
-    // test with a non-merge commit, so the descendants should be rebased onto
-    // the single parent (commit "a"). Then we test with a merge commit, so the
-    // descendants should be rebased onto the two parents.
+    // Descendants of the rebased commit "b" should be rebased onto parents. First we
+    // test with a non-merge commit. Normally, the descendant "c" would still have 2 parents
+    // afterwards: the parent of "b" -- the root commit -- and "a". However,
+    // since the root commit is an ancestor of "a", we don't actually want both to be
+    // parents of the same commit. So, only "a" becomes a parent.
     let stdout = test_env.jj_cmd_success(&repo_path, &["rebase", "-r", "b", "-d", "a"]);
     insta::assert_snapshot!(stdout, @r###"
     Also rebased 3 descendant commits onto parent of rebased commit
-    Working copy now at: b5ce312dacd3 (no description set)
-    Added 0 files, modified 0 files, removed 2 files
+    Working copy now at: e7299ad0c9a7 (no description set)
+    Added 0 files, modified 0 files, removed 1 files
     "###);
     insta::assert_snapshot!(get_log_output(&test_env, &repo_path), @r###"
     @ 
     o d
     o c
     | o b
-    | o a
     |/  
+    o a
     o 
     "###);
     test_env.jj_cmd_success(&repo_path, &["undo"]);
+
+    // Now, let's try moving the merge commit. After, both parents of "c" ("a" and "b"
+    // should become parents of "d".
     let stdout = test_env.jj_cmd_success(&repo_path, &["rebase", "-r", "c", "-d", "root"]);
     insta::assert_snapshot!(stdout, @r###"
     Also rebased 2 descendant commits onto parent of rebased commit
-    Working copy now at: 5e21ebf9fd2b (no description set)
+    Working copy now at: 2d90465bd244 (no description set)
     Added 0 files, modified 0 files, removed 1 files
     "###);
     insta::assert_snapshot!(get_log_output(&test_env, &repo_path), @r###"
@@ -226,6 +230,49 @@ fn test_rebase_single_revision() {
     "###);
 }
 
+#[test]
+fn test_rebase_single_revision_merge_parent() {
+    let test_env = TestEnvironment::default();
+    test_env.jj_cmd_success(test_env.env_root(), &["init", "repo", "--git"]);
+    let repo_path = test_env.env_root().join("repo");
+
+    create_commit(&test_env, &repo_path, "a", &[]);
+    create_commit(&test_env, &repo_path, "b", &[]);
+    create_commit(&test_env, &repo_path, "c", &["b"]);
+    create_commit(&test_env, &repo_path, "d", &["a", "c"]);
+    // Test the setup
+    insta::assert_snapshot!(get_log_output(&test_env, &repo_path), @r###"
+    @ 
+    o   d
+    |\  
+    o | c
+    o | b
+    | o a
+    |/  
+    o 
+    "###);
+
+    // Descendants of the rebased commit should be rebased onto parents, and if
+    // the descendant is a merge commit, it shouldn't forget its other parents.
+    let stdout = test_env.jj_cmd_success(&repo_path, &["rebase", "-r", "c", "-d", "a"]);
+    insta::assert_snapshot!(stdout, @r###"
+    Also rebased 2 descendant commits onto parent of rebased commit
+    Working copy now at: 9b0a69a895b4 (no description set)
+    Added 0 files, modified 0 files, removed 1 files
+    "###);
+    insta::assert_snapshot!(get_log_output(&test_env, &repo_path), @r###"
+    @ 
+    o   d
+    |\  
+    | | o c
+    | |/  
+    o | b
+    | o a
+    |/  
+    o 
+    "###);
+}
+
 #[test]
 fn test_rebase_multiple_destinations() {
     let test_env = TestEnvironment::default();