merged_tree: make tree builder attempt to resolve conflicts

As we discovered in the `jj fix` tests,
`MergedTreeBuilder::write_tree()` doesn't try to resolve conflicts,
not even trivial ones. This patch fixes that.

cli/tests/test_fix_command.rs

@@ -512,7 +512,7 @@ fn test_fix_resolve_conflict() {
     insta::assert_snapshot!(stdout, @"");
     insta::assert_snapshot!(stderr, @r###"
     Fixed 3 commits of 3 checked.
-    Working copy now at: mzvwutvl 98dec555 (conflict) (no description set)
+    Working copy now at: mzvwutvl 669396ce (empty) (no description set)
     Parent commit      : qpvuntsm 3c63716f a | (no description set)
     Parent commit      : kkmpptxz 82703f5e b | (no description set)
     Added 0 files, modified 1 files, removed 0 files

lib/src/merged_tree.rs

@@ -1197,7 +1197,14 @@ impl MergedTreeBuilder {
             MergedTreeId::Merge(base_tree_ids) => base_tree_ids,
         };
         let new_tree_ids = self.write_merged_trees(base_tree_ids, store)?;
-        Ok(MergedTreeId::Merge(new_tree_ids.simplify()))
+        match new_tree_ids.simplify().into_resolved() {
+            Ok(single_tree_id) => Ok(MergedTreeId::resolved(single_tree_id)),
+            Err(tree_id) => {
+                let tree = store.get_root_tree(&MergedTreeId::Merge(tree_id))?;
+                let resolved = tree.resolve()?;
+                Ok(resolved.id())
+            }
+        }
     }
 
     fn write_merged_trees(

lib/tests/test_merged_tree.rs

@@ -23,7 +23,7 @@ use jj_lib::merged_tree::{
     MergedTree, MergedTreeBuilder, MergedTreeVal, TreeDiffIterator, TreeDiffStreamImpl,
 };
 use jj_lib::repo::Repo;
-use jj_lib::repo_path::{RepoPath, RepoPathComponent};
+use jj_lib::repo_path::{RepoPath, RepoPathBuf, RepoPathComponent};
 use jj_lib::tree::merge_trees;
 use pretty_assertions::assert_eq;
 use testutils::{create_single_tree, write_file, TestRepo};
@@ -245,6 +245,28 @@ fn test_from_legacy_tree() {
     assert_eq!(recreated_merged_id, merged_tree.id());
 }
 
+/// Test that a tree built with no changes on top of an add/add conflict gets
+/// resolved.
+#[test]
+fn test_merged_tree_builder_resolves_conflict() {
+    let test_repo = TestRepo::init();
+    let repo = &test_repo.repo;
+    let store = repo.store();
+
+    let path1 = RepoPathBuf::from_internal_string("dir/file");
+    let tree1 = create_single_tree(repo, &[(&path1, "foo")]);
+    let tree2 = create_single_tree(repo, &[(&path1, "bar")]);
+    let tree3 = create_single_tree(repo, &[(&path1, "bar")]);
+
+    let base_tree_id = MergedTreeId::Merge(Merge::from_removes_adds(
+        [tree1.id().clone()],
+        [tree2.id().clone(), tree3.id().clone()],
+    ));
+    let tree_builder = MergedTreeBuilder::new(base_tree_id);
+    let tree_id = tree_builder.write_tree(store).unwrap();
+    assert_eq!(tree_id, MergedTreeId::resolved(tree2.id().clone()));
+}
+
 #[test]
 fn test_path_value_and_entries() {
     let test_repo = TestRepo::init();