rhg-cat: fallback in presence of a fileset

These are unsupported so far.

Differential Revision: https://phab.mercurial-scm.org/D11379

rust/rhg/src/commands/cat.rs

@@ -46,6 +46,11 pub fn run(invocation: &crate::CliInvoca
 
     let mut files = vec![];
     for file in file_args.iter() {
+        if file.starts_with("set:") {
+            let message = "fileset";
+            return Err(CommandError::unsupported(message));
+        }
+
         let normalized = cwd.join(&file);
         // TODO: actually normalize `..` path segments etc?
         let dotted = normalized.components().any(|c| c.as_os_str() == "..");

tests/test-rhg.t

@@ -210,6 +210,11 Fallback with shell path segments
   unsupported feature: `..` or `.` path segment
   [252]
 
+Fallback with filesets
+  $ $NO_FALLBACK rhg cat "set:c or b"
+  unsupported feature: fileset
+  [252]
+
 Requirements
   $ $NO_FALLBACK rhg debugrequirements
   dotencode