conflicts: consider the empty tree a non-legacy tree

Since we no longer depend on legacy trees being preserved when we
build new trees or merge trees, we can consider the root tree a
non-legacy tree.

cli/tests/test_operations.rs

@@ -383,7 +383,7 @@ fn test_op_abandon_ancestors() {
     insta::assert_snapshot!(
         test_env.jj_cmd_success(&repo_path, &["debug", "workingcopy", "--ignore-working-copy"]), @r###"
     Current operation: OperationId("10e856d0579c4aca88972ffea0a515f47f8c09dceaa2b1a5d531c83b04350f7aa64a3eff422def01eebd65df26089ef5e0f925f247a8f929c8cc858d16306e53")
-    Current tree: Legacy(TreeId("4b825dc642cb6eb9a060e54bf8d69288fbee4904"))
+    Current tree: Merge(Resolved(TreeId("4b825dc642cb6eb9a060e54bf8d69288fbee4904")))
     "###);
     insta::assert_snapshot!(test_env.jj_cmd_success(&repo_path, &["op", "log"]), @r###"
     @  10e856d0579c [email protected] 2001-02-03 04:05:09.000 +07:00 - 2001-02-03 04:05:09.000 +07:00
@@ -432,7 +432,7 @@ fn test_op_abandon_ancestors() {
     insta::assert_snapshot!(
         test_env.jj_cmd_success(&repo_path, &["debug", "workingcopy", "--ignore-working-copy"]), @r###"
     Current operation: OperationId("445e93662d714f53fb97e450eb3793a8e1a9a4cca1e329d5a9096e65085fb96d5bb51659531145c8abf0db5da803cd11c4cecdcf9025391bc97fc6a42204a0fe")
-    Current tree: Legacy(TreeId("4b825dc642cb6eb9a060e54bf8d69288fbee4904"))
+    Current tree: Merge(Resolved(TreeId("4b825dc642cb6eb9a060e54bf8d69288fbee4904")))
     "###);
     insta::assert_snapshot!(test_env.jj_cmd_success(&repo_path, &["op", "log"]), @r###"
     @  445e93662d71 [email protected] 2001-02-03 04:05:21.000 +07:00 - 2001-02-03 04:05:21.000 +07:00
@@ -477,7 +477,7 @@ fn test_op_abandon_without_updating_working_copy() {
     insta::assert_snapshot!(
         test_env.jj_cmd_success(&repo_path, &["debug", "workingcopy", "--ignore-working-copy"]), @r###"
     Current operation: OperationId("61aeade2493b190412c61b9b0711025c2cccf95966870a5f7dc2e34ab313fbef64da70d00cab2cd69ad39816faeb80a6bc866927549b84dce6cb5a437e0e515b")
-    Current tree: Legacy(TreeId("4b825dc642cb6eb9a060e54bf8d69288fbee4904"))
+    Current tree: Merge(Resolved(TreeId("4b825dc642cb6eb9a060e54bf8d69288fbee4904")))
     "###);
     insta::assert_snapshot!(
         test_env.jj_cmd_success(&repo_path, &["op", "log", "-l1", "--ignore-working-copy"]), @r###"
@@ -497,7 +497,7 @@ fn test_op_abandon_without_updating_working_copy() {
     insta::assert_snapshot!(
         test_env.jj_cmd_success(&repo_path, &["debug", "workingcopy", "--ignore-working-copy"]), @r###"
     Current operation: OperationId("61aeade2493b190412c61b9b0711025c2cccf95966870a5f7dc2e34ab313fbef64da70d00cab2cd69ad39816faeb80a6bc866927549b84dce6cb5a437e0e515b")
-    Current tree: Legacy(TreeId("4b825dc642cb6eb9a060e54bf8d69288fbee4904"))
+    Current tree: Merge(Resolved(TreeId("4b825dc642cb6eb9a060e54bf8d69288fbee4904")))
     "###);
     insta::assert_snapshot!(
         test_env.jj_cmd_success(&repo_path, &["op", "log", "-l1", "--ignore-working-copy"]), @r###"

lib/src/store.rs

@@ -107,7 +107,7 @@ impl Store {
     }
 
     pub fn empty_merged_tree_id(&self) -> MergedTreeId {
-        MergedTreeId::Legacy(self.backend.empty_tree_id().clone())
+        MergedTreeId::resolved(self.backend.empty_tree_id().clone())
     }
 
     pub fn root_commit(self: &Arc<Self>) -> Commit {

lib/tests/test_merged_tree.rs

@@ -211,15 +211,6 @@ fn test_from_legacy_tree() {
         MergedTreeVal::Resolved(tree.value(dir1_basename))
     );
 
-    // Also test that MergedTreeBuilder can create the same tree by starting from an
-    // empty legacy tree.
-    let mut tree_builder = MergedTreeBuilder::new(store.empty_merged_tree_id());
-    for (path, value) in tree.entries() {
-        tree_builder.set_or_remove(path, Merge::normal(value));
-    }
-    let recreated_legacy_id = tree_builder.write_tree(store).unwrap();
-    assert_eq!(recreated_legacy_id, MergedTreeId::Legacy(tree_id.clone()));
-
     // Create the merged tree by starting from an empty merged tree and adding
     // entries from the merged tree we created before
     let empty_merged_id_builder: MergeBuilder<_> = std::iter::repeat(store.empty_tree_id())