cli: display how many commits were rebased when merging operations (#111)

This involved copying `UnresolvedHeadRepo::resolve()` into the CLI
crate (and modifying it a bit to print number of rebased commit),
which is unfortunate.

lib/src/repo.rs

@@ -303,9 +303,9 @@ impl RepoAtHead {
 }
 
 pub struct UnresolvedHeadRepo {
-    repo_loader: RepoLoader,
-    locked_op_heads: LockedOpHeads,
-    op_heads: Vec<Operation>,
+    pub repo_loader: RepoLoader,
+    pub locked_op_heads: LockedOpHeads,
+    pub op_heads: Vec<Operation>,
 }
 
 impl UnresolvedHeadRepo {

src/commands.rs

@@ -218,8 +218,27 @@ jj init --git-repo=.";
                         ui,
                         "Concurrent modification detected, resolving automatically.",
                     )?;
-                    // TODO: Tell the user how many commits were rebased.
-                    unresolved.resolve(ui.settings())
+                    let base_repo = repo_loader.load_at(&unresolved.op_heads[0]);
+                    // TODO: It may be helpful to print each operation we're merging here
+                    let mut workspace_command = self.for_loaded_repo(ui, workspace, base_repo)?;
+                    let mut tx =
+                        workspace_command.start_transaction("resolve concurrent operations");
+                    for other_op_head in unresolved.op_heads.into_iter().skip(1) {
+                        tx.merge_operation(other_op_head);
+                        let num_rebased = tx.mut_repo().rebase_descendants(ui.settings());
+                        if num_rebased > 0 {
+                            writeln!(
+                                ui,
+                                "Rebased {} descendant commits onto commits rewritten by other \
+                                 operation",
+                                num_rebased
+                            )?;
+                        }
+                    }
+                    let merged_repo = tx.write().leave_unpublished();
+                    unresolved.locked_op_heads.finish(merged_repo.operation());
+                    workspace_command.repo = merged_repo;
+                    return Ok(workspace_command);
                 }
             }
         } else {

tests/test_concurrent_operations.rs

@@ -15,7 +15,7 @@
 use jujutsu::testutils::TestEnvironment;
 
 #[test]
-fn test_concurrent_operations() {
+fn test_concurrent_operation_divergence() {
     let test_env = TestEnvironment::default();
     test_env.jj_cmd_success(test_env.env_root(), &["init", "repo", "--git"]);
     let repo_path = test_env.env_root().join("repo");
@@ -39,3 +39,31 @@ fn test_concurrent_operations() {
     o 
     "###);
 }
+
+#[test]
+fn test_concurrent_operations_auto_rebase() {
+    let test_env = TestEnvironment::default();
+    test_env.jj_cmd_success(test_env.env_root(), &["init", "repo", "--git"]);
+    let repo_path = test_env.env_root().join("repo");
+
+    std::fs::write(repo_path.join("file"), "contents").unwrap();
+    test_env.jj_cmd_success(&repo_path, &["describe", "-m", "initial"]);
+    let stdout = test_env.jj_cmd_success(&repo_path, &["op", "log"]);
+    let op_id_hex = stdout[2..14].to_string();
+
+    test_env.jj_cmd_success(&repo_path, &["describe", "-m", "rewritten"]);
+    test_env.jj_cmd_success(
+        &repo_path,
+        &["new", "--at-op", &op_id_hex, "-m", "new child"],
+    );
+
+    // We should be informed about the concurrent modification
+    let stdout = test_env.jj_cmd_success(&repo_path, &["log", "-T", "commit_id \" \" description"]);
+    insta::assert_snapshot!(stdout, @r###"
+    Concurrent modification detected, resolving automatically.
+    Rebased 1 descendant commits onto commits rewritten by other operation
+    o 4eeb7d76372418118a91c34f09e5e3936f0deeb5 new child
+    @ 14176aeadc0259b2150fc7374969e74b1552a498 rewritten
+    o 0000000000000000000000000000000000000000 
+    "###);
+}