tests/test-gendoc-it.t

perf: use the `perf--` prefix for perf command

This is the one command namespace where they should not be any ambiguity about
command that should be in it. The perf extensions is only adding performance
related command.

so this is a good ground to start putting dash folding to the tests.

Differential Revision: https://phab.mercurial-scm.org/D9516

#require docutils gettext

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