tests: update ChangeIdIndex test to include ancestors in set

I plan to replace `Revset::change_id_index()` by
`Index::change_id_index(heads)`, but one of the tests currently uses a
set of commits that does not include ancestors. This patch updates it
to include ancestors (and changes the set of heads to keep the set
small enough for the test).

lib/tests/test_revset.rs

@@ -2805,30 +2805,26 @@ fn test_change_id_index() {
             .write()
             .unwrap()
     };
-    let commit_1 = commit_with_change_id("aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa");
+    let commit_1 = commit_with_change_id("abbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb");
     let commit_2 = commit_with_change_id("aaaaabbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb");
-    let commit_3 = commit_with_change_id("abbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb");
+    let commit_3 = commit_with_change_id("aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa");
     let commit_4 = commit_with_change_id("bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb");
     let commit_5 = commit_with_change_id("bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb");
 
-    let revset = revset_for_commits(
-        tx.repo(),
-        &[
-            &root_commit,
-            &commit_1,
-            &commit_2,
-            &commit_3,
-            &commit_4,
-            &commit_5,
-        ],
-    );
-    let change_id_index = revset.change_id_index();
+    let index_for_heads = |commits: &[&Commit]| {
+        RevsetExpression::commits(commits.iter().map(|commit| commit.id().clone()).collect())
+            .ancestors()
+            .evaluate_programmatic(tx.repo())
+            .unwrap()
+            .change_id_index()
+    };
+    let change_id_index = index_for_heads(&[&commit_1, &commit_2, &commit_3, &commit_4, &commit_5]);
     let prefix_len =
         |commit: &Commit| change_id_index.shortest_unique_prefix_len(commit.change_id());
     assert_eq!(prefix_len(&root_commit), 1);
-    assert_eq!(prefix_len(&commit_1), 6);
+    assert_eq!(prefix_len(&commit_1), 2);
     assert_eq!(prefix_len(&commit_2), 6);
-    assert_eq!(prefix_len(&commit_3), 2);
+    assert_eq!(prefix_len(&commit_3), 6);
     assert_eq!(prefix_len(&commit_4), 1);
     assert_eq!(prefix_len(&commit_5), 1);
     let resolve_prefix =
@@ -2843,15 +2839,15 @@ fn test_change_id_index() {
     );
     assert_eq!(
         resolve_prefix("aaaaaa"),
-        PrefixResolution::SingleMatch(vec![commit_1.id().clone()])
+        PrefixResolution::SingleMatch(vec![commit_3.id().clone()])
     );
     assert_eq!(
         resolve_prefix("aaaaab"),
         PrefixResolution::SingleMatch(vec![commit_2.id().clone()])
     );
     assert_eq!(
         resolve_prefix("ab"),
-        PrefixResolution::SingleMatch(vec![commit_3.id().clone()])
+        PrefixResolution::SingleMatch(vec![commit_1.id().clone()])
     );
     assert_eq!(
         resolve_prefix("b"),
@@ -2860,32 +2856,35 @@ fn test_change_id_index() {
     // Longer than necessary
     assert_eq!(
         resolve_prefix("aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa"),
-        PrefixResolution::SingleMatch(vec![commit_1.id().clone()])
+        PrefixResolution::SingleMatch(vec![commit_3.id().clone()])
     );
     // No match
     assert_eq!(resolve_prefix("ba"), PrefixResolution::NoMatch);
 
     // Test with a revset containing only some of the commits. We should get shorter
     // prefixes and be able to resolve shorter prefixes.
-    let revset = revset_for_commits(tx.repo(), &[&commit_2, &commit_3]);
-    let change_id_index = revset.change_id_index();
+    let change_id_index = index_for_heads(&[&commit_1, &commit_2]);
     let prefix_len =
         |commit: &Commit| change_id_index.shortest_unique_prefix_len(commit.change_id());
-    assert_eq!(prefix_len(&commit_1), 6);
+    assert_eq!(prefix_len(&commit_1), 2);
     assert_eq!(prefix_len(&commit_2), 2);
-    assert_eq!(prefix_len(&commit_3), 2);
+    assert_eq!(prefix_len(&commit_3), 6);
     let resolve_prefix =
         |prefix: &str| change_id_index.resolve_prefix(&HexPrefix::new(prefix).unwrap());
-    assert_eq!(resolve_prefix("0"), PrefixResolution::NoMatch);
+    assert_eq!(
+        resolve_prefix("0"),
+        PrefixResolution::SingleMatch(vec![root_commit.id().clone()])
+    );
     assert_eq!(
         resolve_prefix("aa"),
         PrefixResolution::SingleMatch(vec![commit_2.id().clone()])
     );
     assert_eq!(
         resolve_prefix("ab"),
-        PrefixResolution::SingleMatch(vec![commit_3.id().clone()])
+        PrefixResolution::SingleMatch(vec![commit_1.id().clone()])
     );
     assert_eq!(resolve_prefix("a"), PrefixResolution::AmbiguousMatch);
+    assert_eq!(resolve_prefix("b"), PrefixResolution::NoMatch);
 }
 
 #[test]