tests/test-gendoc-it.t

revlog: use an explicit config option to enable mmap usage for index

We replace the `experimental.mmapindexthreshold` with two options:

The `storage.revlog.mmap.index` is a boolean option to enable or disable the
feature. The `storage.revlog.mmap.index:size-threshold` is a bytes option that
control when we will be using mmap instead of plain reading.

#require docutils gettext

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