rust-pathauditor: actually populate the `audited_dirs` cache

rust-pathauditor: actually populate the `audited_dirs` cache

I forgot this when rewriting this logic during the review process.

Differential Revision: https://phab.mercurial-scm.org/D8212

--- a/rust/hg-core/src/utils/path_auditor.rs	Wed Mar 04 15:10:03 2020 +0100
+++ b/rust/hg-core/src/utils/path_auditor.rs	Wed Mar 04 15:12:08 2020 +0100
@@ -117,6 +117,7 @@
                 continue;
             }
             self.check_filesystem(&prefix, &path)?;
+            self.audited_dirs.insert(prefix.to_owned());
         }
 
         self.audited.insert(path.to_owned());