lib id_prefix: look for divergent changes outside short prefix set

Fixes #2476.

Previously, if there was a change id match within the short prefix
lookup set, `jj` would not look for commits with that same change id
outside the short prefix set. So, it wouldn't find the conflicted
commits for a commit with a divergent (AKA conflicted) change id.

CHANGELOG.md

@@ -82,6 +82,8 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
   to-be-pushed commit has conflicts, or has no description / committer / author
   set. [#3029](https://github.com/martinvonz/jj/issues/3029)
 
+* `jj` will look for divergent changes outside the short prefix set even if it finds the change id inside the short prefix set. [#2476](https://github.com/martinvonz/jj/issues/2476)
+
 ## [0.18.0] - 2024-06-05
 
 ### Breaking changes

lib/src/id_prefix.rs

@@ -168,13 +168,14 @@ impl IdPrefixContext {
         prefix: &HexPrefix,
     ) -> PrefixResolution<Vec<CommitId>> {
         if let Some(indexes) = self.disambiguation_indexes(repo) {
-            let resolution = indexes.change_index.resolve_prefix_with(
-                &*indexes.commit_change_ids,
-                prefix,
-                |(commit_id, _)| commit_id.clone(),
-            );
-            if let PrefixResolution::SingleMatch((_, ids)) = resolution {
-                return PrefixResolution::SingleMatch(ids);
+            let resolution = indexes
+                .change_index
+                .resolve_prefix_to_key(&*indexes.commit_change_ids, prefix);
+            if let PrefixResolution::SingleMatch(change_id) = resolution {
+                // There may be more commits with this change id outside the narrower sets.
+                return PrefixResolution::SingleMatch(repo.resolve_change_id(&change_id).expect(
+                    "Change ids present in narrower search set should be present globally.",
+                ));
             }
         }
         repo.resolve_change_id_prefix(prefix)

lib/tests/test_id_prefix.rs

@@ -365,12 +365,19 @@ fn test_id_prefix_divergent() {
         c.shortest_change_prefix_len(repo.as_ref(), second_commit.change_id()),
         1
     );
-    // Short prefix does not find the first commit (as intended).
-    // TODO(#2476): BUG: Looking up the divergent commits by their change id prefix
-    // only finds the id within the lookup set.
+    // This tests two issues, both important:
+    // - We find both commits with the same change id, even though
+    // `third_commit_divergent_with_second` is not in the short prefix set
+    // (#2476).
+    // - The short prefix set still works: we do *not* find the first commit and the
+    //   match is not ambiguous, even though the first commit's change id would also
+    //   match the prefix.
     assert_eq!(
         c.resolve_change_prefix(repo.as_ref(), &prefix("a")),
-        SingleMatch(vec![second_commit.id().clone()])
+        SingleMatch(vec![
+            second_commit.id().clone(),
+            third_commit_divergent_with_second.id().clone()
+        ])
     );
 
     // We can still resolve commits outside the set