rust-status: fix thread count ceiling

This was forcing 16 threads instead of creating a ceiling, which is wrong
when either the available parallelism of the platform is lower or when
the user wants to set it explicitly (like we do in `run-tests.py`)

rust/hg-core/src/dirstate_tree/status.rs

@@ -56,7 +56,7 b" pub fn status<'dirstate>("
     // instantiated in `rhg` or some other caller.
     // TODO find the underlying cause and fix it, then remove this.
     rayon::ThreadPoolBuilder::new()
-        .num_threads(16)
+        .num_threads(16.min(rayon::current_num_threads()))
         .build_global()
         .ok();