workspace: use cwd for printing relative path

The user probably would expect the path to be relative to their current
directory rather than the workspace root. For instance, if the user is
in a child directory and runs `jj workspace add ../../name`, then they
might be surprised if we printed "../name" instead of "../../name".

cli/src/commands/workspace.rs

@@ -169,8 +169,7 @@ fn cmd_workspace_add(
     writeln!(
         ui.status(),
         "Created workspace in \"{}\"",
-        file_util::relative_path(old_workspace_command.workspace_root(), &destination_path)
-            .display()
+        file_util::relative_path(command.cwd(), &destination_path).display()
     )?;
 
     // Copy sparse patterns from workspace where the command was run

cli/tests/test_workspaces.rs

@@ -252,6 +252,42 @@ fn test_workspaces_add_workspace_multiple_revisions() {
     "###);
 }
 
+#[test]
+fn test_workspaces_add_workspace_from_subdir() {
+    let test_env = TestEnvironment::default();
+    test_env.jj_cmd_ok(test_env.env_root(), &["git", "init", "main"]);
+    let main_path = test_env.env_root().join("main");
+    let subdir_path = main_path.join("subdir");
+    let secondary_path = test_env.env_root().join("secondary");
+
+    std::fs::create_dir(&subdir_path).unwrap();
+    std::fs::write(subdir_path.join("file"), "contents").unwrap();
+    test_env.jj_cmd_ok(&main_path, &["commit", "-m", "initial"]);
+
+    let stdout = test_env.jj_cmd_success(&main_path, &["workspace", "list"]);
+    insta::assert_snapshot!(stdout, @r###"
+    default: rlvkpnrz e1038e77 (empty) (no description set)
+    "###);
+
+    // Create workspace while in sub-directory of current workspace
+    let (stdout, stderr) =
+        test_env.jj_cmd_ok(&subdir_path, &["workspace", "add", "../../secondary"]);
+    insta::assert_snapshot!(stdout.replace('\\', "/"), @"");
+    insta::assert_snapshot!(stderr.replace('\\', "/"), @r###"
+    Created workspace in "../../secondary"
+    Working copy now at: rzvqmyuk 7ad84461 (empty) (no description set)
+    Parent commit      : qpvuntsm a3a43d9e initial
+    Added 1 files, modified 0 files, removed 0 files
+    "###);
+
+    // Both workspaces show up when we list them
+    let stdout = test_env.jj_cmd_success(&secondary_path, &["workspace", "list"]);
+    insta::assert_snapshot!(stdout, @r###"
+    default: rlvkpnrz e1038e77 (empty) (no description set)
+    secondary: rzvqmyuk 7ad84461 (empty) (no description set)
+    "###);
+}
+
 /// Test making changes to the working copy in a workspace as it gets rewritten
 /// from another workspace
 #[test]