revset: clarify error about missing working-copy commit for workspace

The error message that says something like 'Workspace "default"
doesn't have a working copy' confused me when I saw it. The problem
it's describing is that the repo view doesn't have a working-copy
commit for the given workspace id. Saying "working-copy commit"
instead of "working copy" hopefully clarifies it a bit.

cli/tests/test_workspaces.rs

@@ -643,7 +643,7 @@ fn test_workspaces_forget() {
     // Revision "@" cannot be used
     let stderr = test_env.jj_cmd_failure(&main_path, &["log", "-r", "@"]);
     insta::assert_snapshot!(stderr, @r###"
-    Error: Workspace "default" doesn't have a working copy
+    Error: Workspace "default" doesn't have a working-copy commit
     "###);
 
     // Try to add back the workspace

lib/src/revset.rs

@@ -53,7 +53,7 @@ pub enum RevsetResolutionError {
         name: String,
         candidates: Vec<String>,
     },
-    #[error("Workspace \"{name}\" doesn't have a working copy")]
+    #[error("Workspace \"{name}\" doesn't have a working-copy commit")]
     WorkspaceMissingWorkingCopy { name: String },
     #[error("An empty string is not a valid revision")]
     EmptyString,