ignore: improve multithreading heuristic

This copies the one found in ripgrep.

See also:
71d71d2d98/crates/core/flags/hiargs.rs (L172)

Closes #2854, Closes #2856

crates/ignore/src/walk.rs

@@ -1305,7 +1305,7 @@ impl WalkParallel {
 
     fn threads(&self) -> usize {
         if self.threads == 0 {
-            2
+            std::thread::available_parallelism().map_or(1, |n| n.get()).min(12)
         } else {
             self.threads
         }