rust-status: collect traversed directories if required...
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
