rhg: make it possible to opt out of `rhg cat`

The reason an opt-out is needed is that there are still behavior
differences between `rhg cat` and `hg cat`:

- it does not interpret relative paths correctly
- it does not interpret patterns correctly, e.g. 're:foobar$' would be interpreted as a verbatim filename
- it does not implement the correct semantics of relpath matcher: if given a directory, `hg` concatenates all files in this directory, while `rhg` simply complains

Differential Revision: https://phab.mercurial-scm.org/D11723

rust/rhg/src/commands/cat.rs

@@ -33,6 +33,15 b" pub fn args() -> clap::App<'static, 'sta"
 
 #[timed]
 pub fn run(invocation: &crate::CliInvocation) -> Result<(), CommandError> {
+    let cat_enabled_default = true;
+    let cat_enabled = invocation.config.get_option(b"rhg", b"cat")?;
+    if !cat_enabled.unwrap_or(cat_enabled_default) {
+        return Err(CommandError::unsupported(
+            "cat is disabled in rhg (enable it with 'rhg.cat = true' \
+            or enable fallback with 'rhg.on-unsupported = fallback')",
+        ));
+    }
+
     let rev = invocation.subcommand_args.value_of("rev");
     let file_args = match invocation.subcommand_args.values_of("files") {
         Some(files) => files.collect(),