Index.get_indexer_non_unique misbehaves when passed a CategoricalIndex

[toobaz]

Code Sample, a copy-pastable example if possible

In [2]: idx = pd.CategoricalIndex(['a', 'a', 'b', 'b', 'c', 'a'], categories=['c', 'a', 'b', 'e
   ...: '], dtype='category')

In [3]: key = ['a', 'b']

In [4]: idx.get_indexer_non_unique(key)
Out[4]: (array([0, 1, 5, 2, 3]), array([], dtype=int64))

In [5]: idx.get_indexer_non_unique(pd.CategoricalIndex(key, categories=idx.categories))
Out[5]: (array([ 4,  0,  1,  5,  2,  3, -1]), array([3]))

In [6]: pd.Index(['a', 'a', 'b', 'b', 'c', 'a']).get_indexer_non_unique(pd.CategoricalIndex(key
   ...: , categories=idx.categories))
Out[6]: (array([-1, -1]), array([0, 1]))

Problem description

Out[5] and Out[6] are both wrong, and apparently in pretty different ways.

Expected Output

Out[3]

Output of pd.show_versions()

INSTALLED VERSIONS

commit: defdb34
python: 3.5.3.final.0
python-bits: 64
OS: Linux
OS-release: 4.9.0-6-amd64
machine: x86_64
processor:
byteorder: little
LC_ALL: None
LANG: it_IT.UTF-8
LOCALE: it_IT.UTF-8

pandas: 0.24.0.dev0+88.gdefdb34ba
pytest: 3.5.0
pip: 9.0.1
setuptools: 39.2.0
Cython: 0.25.2
numpy: 1.14.3
scipy: 0.19.0
pyarrow: None
xarray: None
IPython: 6.2.1
sphinx: 1.5.6
patsy: 0.5.0
dateutil: 2.7.3
pytz: 2018.4
blosc: None
bottleneck: 1.2.0dev
tables: 3.3.0
numexpr: 2.6.1
feather: 0.3.1
matplotlib: 2.2.2.post1153+gff6786446
openpyxl: 2.3.0
xlrd: 1.0.0
xlwt: 1.3.0
xlsxwriter: 0.9.6
lxml: 4.1.1
bs4: 4.5.3
html5lib: 0.999999999
sqlalchemy: 1.0.15
pymysql: None
psycopg2: None
jinja2: 2.10
s3fs: None
fastparquet: None
pandas_gbq: None
pandas_datareader: 0.2.1