cli: warn explicit paths not exist in either of diff trees

Maybe we can optimize it to check paths during diffing, but I think it's okay
to add extra lookup cost at the end. The size of the path arguments is usually
small.

Closes #505

cli/src/commands/diff.rs

@@ -12,6 +12,7 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
+use itertools::Itertools as _;
 use jj_lib::rewrite::merge_commit_trees;
 use tracing::instrument;
 
@@ -76,9 +77,8 @@ pub(crate) fn cmd_diff(
         from_tree = merge_commit_trees(workspace_command.repo().as_ref(), &parents)?;
         to_tree = commit.tree()?
     }
-    let matcher = workspace_command
-        .parse_file_patterns(&args.paths)?
-        .to_matcher();
+    let fileset_expression = workspace_command.parse_file_patterns(&args.paths)?;
+    let matcher = fileset_expression.to_matcher();
     let diff_formats = diff_formats_for(command.settings(), &args.format)?;
     ui.request_pager();
     show_diff(
@@ -90,5 +90,19 @@ pub(crate) fn cmd_diff(
         matcher.as_ref(),
         &diff_formats,
     )?;
+
+    let unmatched_explicit_paths = fileset_expression
+        .explicit_paths()
+        .filter(|path| from_tree.path_value(path).is_absent())
+        .filter(|path| to_tree.path_value(path).is_absent())
+        .map(|path| workspace_command.format_file_path(path))
+        .collect_vec();
+    if !unmatched_explicit_paths.is_empty() {
+        writeln!(
+            ui.warning_default(),
+            "No matching entries for paths: {}",
+            unmatched_explicit_paths.join(", ")
+        )?;
+    }
     Ok(())
 }

cli/tests/test_diff_command.rs

@@ -149,6 +149,33 @@ fn test_diff_basic() {
     file3 | 1 +
     3 files changed, 2 insertions(+), 1 deletion(-)
     "###);
+
+    // Unmatched paths should be warned
+    let (stdout, stderr) = test_env.jj_cmd_ok(
+        test_env.env_root(),
+        &[
+            "diff",
+            "-Rrepo",
+            "-s",
+            "repo",       // matches directory
+            "repo/file1", // deleted in to_tree, but exists in from_tree
+            "repo/x",
+            "repo/y/z",
+        ],
+    );
+    insta::assert_snapshot!(stdout.replace('\\', "/"), @r###"
+    D repo/file1
+    M repo/file2
+    A repo/file3
+    "###);
+    insta::assert_snapshot!(stderr.replace('\\', "/"), @r###"
+    Warning: No matching entries for paths: repo/x, repo/y/z
+    "###);
+
+    // Unmodified paths shouldn't be warned
+    let (stdout, stderr) = test_env.jj_cmd_ok(&repo_path, &["diff", "-s", "--from=@", "file2"]);
+    insta::assert_snapshot!(stdout, @"");
+    insta::assert_snapshot!(stderr, @"");
 }
 
 #[test]