ischildnode can be quite hot in some cases involving many fixtures.
However it is always used in a way that the nodeid is constant and the
baseid is iterated. So we can save work by pre-computing the parents of
the nodeid and use a simple containment test.
The `_getautousenames` function has the same stuff open-coded, so change
it to use the new function as well.
Ref: https://github.com/pytest-dev/pytest/issues/4321#issuecomment-436951894
Hardens some of the not many tests affected by this:
1. `testing/test_session.py::test_rootdir_option_arg` displayed:
> root/test_rootdir_option_arg2/test_rootdir_option_arg.py
2. `test_cmdline_python_namespace_package` displayed "hello/" prefix for:
> hello/test_hello.py::test_hello
> hello/test_hello.py::test_other
Ran Benita
Simon K
crricks
Bruno Oliveira
Ronny Pfannschmidt
Anthony Sottile
Daniel Hahler
Ronny Pfannschmidt
Tom Dalton