rust-status: use bare hg status fastpath from Python
This change also adds a test case for subrepos.
Repeating the benchmark information from the `hg-core` commit:
On the Netbeans repository:
C: 840ms
Rust+C: 556ms
Mozilla Central with the one pattern that causes a fallback removed:
C: 2.315s
Rust+C: 1.700s
Differential Revision: https://phab.mercurial-scm.org/D7931
Hello, Foobar
[b'Hello, Fo, =;:<o', b'Hello, Bar']