ignore: add 'same_file_system' option

This commit adds a 'same_file_system' option to the walk builder. For
single threaded walking, it defers to the walkdir crate, which has the
same option. The bulk of this commit implements this flag for the parallel
walker. We add one very feeble test for this.

The parallel walker is now officially a complete mess.

Closes #321

complete/_rg

@@ -81,6 +81,10 @@ _rg() {
     {-H,--with-filename}'[show file name for matches]'
     "--no-filename[don't show file name for matches]"
 
+    + '(file-system)' # File system options
+    "--one-file-system[don't descend into directories on other file systems]"
+    $no'--no-one-file-system[descend into directories on other file systems]'
+
     + '(fixed)' # Fixed-string options
     {-F,--fixed-strings}'[treat pattern as literal string instead of regular expression]'
     $no"--no-fixed-strings[don't treat pattern as literal string]"