cli: run "op log" without loading repo or merging concurrent ops

When debugging behavior of badly-GCed repos, I find it's annoying that "op log"
fails because the index can't be loaded. Since "op log" doesn't need a repo, I
think it's better to display the exact op-heads state without merging.

cli/src/commands/operation.rs

@@ -132,26 +132,45 @@ fn cmd_op_log(
     command: &CommandHelper,
     args: &OperationLogArgs,
 ) -> Result<(), CommandError> {
-    let workspace_command = command.workspace_helper(ui)?;
-    let repo = workspace_command.repo();
-    let head_op = repo.operation().clone();
-    let head_op_id = head_op.id().clone();
+    // Don't load the repo so that the operation history can be inspected even
+    // with a corrupted repo state. For example, you can find the first bad
+    // operation id to be abandoned.
+    let workspace = command.load_workspace()?;
+    let repo_loader = workspace.repo_loader();
+    let head_op_str = &command.global_args().at_operation;
+    let head_ops = if head_op_str == "@" {
+        // If multiple head ops can't be resolved without merging, let the
+        // current op be empty. Beware that resolve_op_for_load() will eliminate
+        // redundant heads whereas get_current_head_ops() won't.
+        let current_op = op_walk::resolve_op_for_load(repo_loader, head_op_str).ok();
+        if let Some(op) = current_op {
+            vec![op]
+        } else {
+            op_walk::get_current_head_ops(
+                repo_loader.op_store(),
+                repo_loader.op_heads_store().as_ref(),
+            )?
+        }
+    } else {
+        vec![op_walk::resolve_op_for_load(repo_loader, head_op_str)?]
+    };
+    let current_op_id = match &*head_ops {
+        [op] => Some(op.id()),
+        _ => None,
+    };
 
     let template_string = match &args.template {
         Some(value) => value.to_owned(),
         None => command.settings().config().get_string("templates.op_log")?,
     };
-    let template = operation_templater::parse(
-        Some(repo.op_id()),
-        &template_string,
-        workspace_command.template_aliases_map(),
-    )?;
+    let template_aliases = command.load_template_aliases(ui)?;
+    let template = operation_templater::parse(current_op_id, &template_string, &template_aliases)?;
     let with_content_format = LogContentFormat::new(ui, command.settings())?;
 
     ui.request_pager();
     let mut formatter = ui.stdout_formatter();
     let formatter = formatter.as_mut();
-    let iter = op_walk::walk_ancestors(&[head_op]).take(args.limit.unwrap_or(usize::MAX));
+    let iter = op_walk::walk_ancestors(&head_ops).take(args.limit.unwrap_or(usize::MAX));
     if !args.no_graph {
         let mut graph = get_graphlog(command.settings(), formatter.raw());
         let default_node_symbol = graph.default_node_symbol().to_owned();
@@ -161,7 +180,7 @@ fn cmd_op_log(
             for id in op.parent_ids() {
                 edges.push(Edge::direct(id.clone()));
             }
-            let is_head_op = op.id() == &head_op_id;
+            let is_current_op = Some(op.id()) == current_op_id;
             let mut buffer = vec![];
             with_content_format.write_graph_text(
                 ui.new_formatter(&mut buffer).as_mut(),
@@ -173,7 +192,7 @@ fn cmd_op_log(
             if !buffer.ends_with(b"\n") {
                 buffer.push(b'\n');
             }
-            let node_symbol = if is_head_op {
+            let node_symbol = if is_current_op {
                 "@"
             } else {
                 &default_node_symbol

cli/tests/test_concurrent_operations.rs

@@ -32,6 +32,21 @@ fn test_concurrent_operation_divergence() {
         &["describe", "-m", "message 2", "--at-op", "@-"],
     );
 
+    // "op log" doesn't merge the concurrent operations
+    let stdout = test_env.jj_cmd_success(&repo_path, &["op", "log"]);
+    insta::assert_snapshot!(stdout, @r###"
+    ◉  94fce7319d45 test-username@host.example.com 2001-02-03 04:05:09.000 +07:00 - 2001-02-03 04:05:09.000 +07:00
+    │  describe commit 230dd059e1b059aefc0da06a2e5a7dbf22362f22
+    │  args: jj describe -m 'message 2' --at-op @-
+    │ ◉  15e35d1e9190 test-username@host.example.com 2001-02-03 04:05:08.000 +07:00 - 2001-02-03 04:05:08.000 +07:00
+    ├─╯  describe commit 230dd059e1b059aefc0da06a2e5a7dbf22362f22
+    │    args: jj describe -m 'message 1'
+    ◉  19b8089fc78b test-username@host.example.com 2001-02-03 04:05:07.000 +07:00 - 2001-02-03 04:05:07.000 +07:00
+    │  add workspace 'default'
+    ◉  f1c462c494be test-username@host.example.com 2001-02-03 04:05:07.000 +07:00 - 2001-02-03 04:05:07.000 +07:00
+       initialize repo
+    "###);
+
     // We should be informed about the concurrent modification
     let (stdout, stderr) = test_env.jj_cmd_ok(&repo_path, &["log", "-T", "description"]);
     insta::assert_snapshot!(stdout, @r###"

lib/src/op_walk.rs

@@ -158,7 +158,7 @@ fn resolve_single_op_from_store(
 
 /// Loads the current head operations. The returned operations may contain
 /// redundant ones which are ancestors of the other heads.
-fn get_current_head_ops(
+pub fn get_current_head_ops(
     op_store: &Arc<dyn OpStore>,
     op_heads_store: &dyn OpHeadsStore,
 ) -> OpStoreResult<Vec<Operation>> {