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
$ hg init
$ touch a
$ unset HGUSER
$ echo "[ui]" >> .hg/hgrc
$ echo "username= foo" >> .hg/hgrc
$ echo " bar1" >> .hg/hgrc
$ hg ci -Am m
adding a
abort: username 'foo\nbar1' contains a newline
[255]
$ rm .hg/hgrc
$ HGUSER=`(echo foo; echo bar2)` hg ci -Am m
adding a
abort: username 'foo\nbar2' contains a newline
[255]
$ hg ci -Am m -u "`(echo foo; echo bar3)`"
adding a
transaction abort!
rollback completed
abort: username 'foo\nbar3' contains a newline!
[255]