rust-status: collect traversed directories if required Some commands (`hg purge` notably) register the `traversedir` callback on their matcher to run said callback every time a directory is traversed. This is the first of three patches, further broadening Rust support for status. Unfortunately, there is no way around collecting a full `Vec` (or any other owned datastructure, like a radix tree) and pushing it back up the Python layer since keeping the Python callback in a closure would mean giving up multithreading because of the GIL, which is obviously unacceptable. Performance is still a lot better than the Python+C path. Running `hg clean/purge` on Netbeans' repo (100k files): ``` | No-op | 30% unknown -------------------------- Rust | 1.0s | 1.67s C | 2.0s | 2.87s ``` Differential Revision: https://phab.mercurial-scm.org/D8518