op show: accept root operation by mapping empty parents to root

I'm thinking of adding an option to embed operation diffs in "op log", and
"op log" shouldn't fail at the root operation. Let's make "op diff"/"show"
also work for consistency.

cli/src/commands/operation/diff.rs

@@ -40,7 +40,6 @@ use crate::cli_util::short_change_hash;
 use crate::cli_util::short_operation_hash;
 use crate::cli_util::CommandHelper;
 use crate::cli_util::LogContentFormat;
-use crate::command_error::user_error;
 use crate::command_error::CommandError;
 use crate::diff_util::DiffFormatArgs;
 use crate::diff_util::DiffRenderer;
@@ -93,9 +92,6 @@ pub fn cmd_op_diff(
     } else {
         to_op = workspace_command.resolve_single_op(args.operation.as_deref().unwrap_or("@"))?;
         let to_op_parents: Vec<_> = to_op.parents().try_collect()?;
-        if to_op_parents.is_empty() {
-            return Err(user_error("Cannot diff operation with no parents"));
-        }
         from_op = repo_loader.merge_operations(command.settings(), to_op_parents, None)?;
     }
     let graph_style = GraphStyle::from_settings(command.settings())?;

cli/src/commands/operation/show.rs

@@ -17,7 +17,6 @@ use itertools::Itertools;
 use super::diff::show_op_diff;
 use crate::cli_util::CommandHelper;
 use crate::cli_util::LogContentFormat;
-use crate::command_error::user_error;
 use crate::command_error::CommandError;
 use crate::diff_util::DiffFormatArgs;
 use crate::graphlog::GraphStyle;
@@ -55,9 +54,6 @@ pub fn cmd_op_show(
     let repo_loader = &repo.loader();
     let op = workspace_command.resolve_single_op(&args.operation)?;
     let parents: Vec<_> = op.parents().try_collect()?;
-    if parents.is_empty() {
-        return Err(user_error("Cannot show the root operation"));
-    }
     let parent_op = repo_loader.merge_operations(command.settings(), parents, None)?;
     let parent_repo = repo_loader.load_at(&parent_op)?;
     let repo = repo_loader.load_at(&op)?;

cli/tests/test_operations.rs

@@ -1211,9 +1211,9 @@ fn test_op_show() {
     "###);
 
     // The root operation is empty.
-    let stderr = test_env.jj_cmd_failure(&repo_path, &["op", "show", "0000000"]);
-    insta::assert_snapshot!(&stderr, @r###"
-    Error: Cannot show the root operation
+    let stdout = test_env.jj_cmd_success(&repo_path, &["op", "show", "0000000"]);
+    insta::assert_snapshot!(&stdout, @r###"
+    000000000000 root()
     "###);
 
     // Showing the latest operation.

lib/src/repo.rs

@@ -753,8 +753,8 @@ impl RepoLoader {
         Arc::new(repo)
     }
 
-    /// Merges the given `operations` into a single operation.
-    /// Assumes that there is at least one operation.
+    /// Merges the given `operations` into a single operation. Returns the root
+    /// operation if the `operations` is empty.
     pub fn merge_operations(
         &self,
         settings: &UserSettings,
@@ -763,7 +763,11 @@ impl RepoLoader {
     ) -> Result<Operation, RepoLoaderError> {
         let num_operations = operations.len();
         let mut operations = operations.into_iter();
-        let base_op = operations.next().unwrap();
+        let Some(base_op) = operations.next() else {
+            let id = self.op_store.root_operation_id();
+            let data = self.op_store.read_operation(id)?;
+            return Ok(Operation::new(self.op_store.clone(), id.clone(), data));
+        };
         let final_op = if num_operations > 1 {
             let base_repo = self.load_at(&base_op)?;
             let mut tx = base_repo.start_transaction(settings);
@@ -789,6 +793,7 @@ impl RepoLoader {
         op_heads: Vec<Operation>,
         user_settings: &UserSettings,
     ) -> Result<Operation, RepoLoaderError> {
+        assert!(!op_heads.is_empty());
         self.merge_operations(
             user_settings,
             op_heads,