Rework parallelism in directory iterator.

Previously, ignore::WalkParallel would invoke the callback for all
*explicitly* given file paths in a single thread, which effectively
meant that `rg pattern foo bar baz ...` didn't actually search foo, bar
and baz in parallel.

The code was structured that way to avoid spinning up workers if no
directory paths were given. The original intention was probably to have
a separate pool of threads responsible for searching, but ripgrep ended
up just reusing the ignore::WalkParallel workers themselves for searching,
and thereby subjected to its sub-par performance in this case.

The code has been restructured so that file paths are sent to the workers,
which brings back parallelism.

Fixes #226

src/args.rs

@@ -1,3 +1,4 @@
+use std::cmp;
 use std::env;
 use std::io;
 use std::path::{Path, PathBuf};
@@ -376,7 +377,7 @@ impl RawArgs {
             };
         let threads =
             if self.flag_threads == 0 {
-                num_cpus::get()
+                cmp::min(12, num_cpus::get())
             } else {
                 self.flag_threads
             };