tests/test-gendoc-sv.t

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

#require docutils gettext

  $ $TESTDIR/check-gendoc sv
  checking for parse errors