Fix groupby shift/diff/fill after selecting from a GroupBy

[shwina]

Fixes #9969

Due to a bug in GroupBy.__getitem__, selecting a column of a GroupBy and then doing a shift, diff, or fill operation would result in the operation being performed on the wrong values. This PR fixes GroupBy.__getitem__ so we now have the right behaviour.

[isVoid]

thnx!

[codecov]

Codecov Report

Merging #9984 (064b681) into branch-22.02 (967a333) will decrease coverage by 0.03%.
The diff coverage is n/a.

❗ Current head 064b681 differs from pull request most recent head d136cb4. Consider uploading reports for the commit d136cb4 to get more accurate results

@@               Coverage Diff                @@
##           branch-22.02    #9984      +/-   ##
================================================
- Coverage         10.49%   10.45%   -0.04%     
================================================
  Files               119      119              
  Lines             20305    20414     +109     
================================================
+ Hits               2130     2134       +4     
- Misses            18175    18280     +105     

Impacted Files	Coverage Δ
python/custreamz/custreamz/kafka.py	29.16% <0.00%> (-0.63%)	⬇️
python/dask_cudf/dask_cudf/sorting.py	92.30% <0.00%> (-0.61%)	⬇️
python/cudf/cudf/__init__.py	0.00% <0.00%> (ø)
python/cudf/cudf/core/frame.py	0.00% <0.00%> (ø)
python/cudf/cudf/core/index.py	0.00% <0.00%> (ø)
python/cudf/cudf/io/parquet.py	0.00% <0.00%> (ø)
python/cudf/cudf/core/series.py	0.00% <0.00%> (ø)
python/cudf/cudf/utils/utils.py	0.00% <0.00%> (ø)
python/cudf/cudf/utils/dtypes.py	0.00% <0.00%> (ø)
python/cudf/cudf/utils/ioutils.py	0.00% <0.00%> (ø)
... and 19 more

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[galipremsagar]

@gpucibot merge