conflicts: fix off-by-one error in materialize_merge_result()

This should fix #1304. I think the added test simulates the behavior of
multiple rebase conflicts, but I don't have expertise around this.

add_index could be replaced with a peekable iterator, but the iterator version
wouldn't be as readable as the current implementation.

lib/src/conflicts.rs

@@ -210,11 +210,9 @@ pub fn materialize_merge_result(
                             // Check if the diff against the next positive term is better. Since
                             // we want to preserve the order of the terms, we don't match against
                             // any later positive terms.
-                            if add_index < adds.len() {
+                            if let Some(right2) = adds.get(add_index + 1) {
-                                let diff2 = Diff::for_tokenizer(
+                                let diff2 =
-                                    &[&left, &adds[add_index + 1]],
+                                    Diff::for_tokenizer(&[&left, right2], &find_line_ranges)
-                                    &find_line_ranges,
-                                )
                                         .hunks()
                                         .collect_vec();
                                 if diff_size(&diff2) < diff_size(&diff1) {

lib/tests/test_conflicts.rs

@@ -114,6 +114,137 @@ line 5
     );
 }
 
+#[test]
+fn test_materialize_conflict_multi_rebase_conflicts() {
+    let test_repo = TestRepo::init(false);
+    let store = test_repo.repo.store();
+
+    // Create changes (a, b, c) on top of the base, and linearize them.
+    let path = RepoPath::from_internal_string("file");
+    let base_id = testutils::write_file(
+        store,
+        &path,
+        "line 1
+line 2 base
+line 3
+",
+    );
+    let a_id = testutils::write_file(
+        store,
+        &path,
+        "line 1
+line 2 a.1
+line 2 a.2
+line 2 a.3
+line 3
+",
+    );
+    let b_id = testutils::write_file(
+        store,
+        &path,
+        "line 1
+line 2 b.1
+line 2 b.2
+line 3
+",
+    );
+    let c_id = testutils::write_file(
+        store,
+        &path,
+        "line 1
+line 2 c.1
+line 3
+",
+    );
+
+    // The order of (a, b, c) should be preserved. For all cases, the "a" side
+    // should be a snapshot.
+    let conflict = Conflict {
+        removes: vec![file_conflict_term(&base_id), file_conflict_term(&base_id)],
+        adds: vec![
+            file_conflict_term(&a_id),
+            file_conflict_term(&b_id),
+            file_conflict_term(&c_id),
+        ],
+    };
+    insta::assert_snapshot!(
+        &materialize_conflict_string(store, &path, &conflict),
+        @r###"
+    line 1
+    <<<<<<<
+    +++++++
+    line 2 a.1
+    line 2 a.2
+    line 2 a.3
+    %%%%%%%
+    -line 2 base
+    +line 2 b.1
+    +line 2 b.2
+    %%%%%%%
+    -line 2 base
+    +line 2 c.1
+    >>>>>>>
+    line 3
+    "###
+    );
+    let conflict = Conflict {
+        removes: vec![file_conflict_term(&base_id), file_conflict_term(&base_id)],
+        adds: vec![
+            file_conflict_term(&c_id),
+            file_conflict_term(&b_id),
+            file_conflict_term(&a_id),
+        ],
+    };
+    insta::assert_snapshot!(
+        &materialize_conflict_string(store, &path, &conflict),
+        @r###"
+    line 1
+    <<<<<<<
+    %%%%%%%
+    -line 2 base
+    +line 2 c.1
+    %%%%%%%
+    -line 2 base
+    +line 2 b.1
+    +line 2 b.2
+    +++++++
+    line 2 a.1
+    line 2 a.2
+    line 2 a.3
+    >>>>>>>
+    line 3
+    "###
+    );
+    let conflict = Conflict {
+        removes: vec![file_conflict_term(&base_id), file_conflict_term(&base_id)],
+        adds: vec![
+            file_conflict_term(&c_id),
+            file_conflict_term(&a_id),
+            file_conflict_term(&b_id),
+        ],
+    };
+    insta::assert_snapshot!(
+        &materialize_conflict_string(store, &path, &conflict),
+        @r###"
+    line 1
+    <<<<<<<
+    %%%%%%%
+    -line 2 base
+    +line 2 c.1
+    +++++++
+    line 2 a.1
+    line 2 a.2
+    line 2 a.3
+    %%%%%%%
+    -line 2 base
+    +line 2 b.1
+    +line 2 b.2
+    >>>>>>>
+    line 3
+    "###
+    );
+}
+
 #[test]
 fn test_materialize_parse_roundtrip() {
     let test_repo = TestRepo::init(false);