Fix bug where df.loc resulting in single row could give wrong index

[eriknw]

Description

Fixes #11930

I can't figure out the purpose of these lines, so let's try removing them and run CI.

I haven't followed git blame back far enough to know the full story of these lines, but they originate at least three years ago:
https://github.com/rapidsai/cudf/pull/2208/files#diff-5f58cf9dfe537ce53c6481f690ba66ff10807da04ad82df1c79c6d112d19c08b

Checklist

• I am familiar with the Contributing Guidelines.
• New or existing tests cover these changes.
• The documentation is up to date with these changes.

[codecov]

Codecov Report

Base: 87.40% // Head: 88.12% // Increases project coverage by +0.71% 🎉

Coverage data is based on head (0382688) compared to base (f72c4ce).
Patch has no changes to coverable lines.

❗ Current head 0382688 differs from pull request most recent head 7eb934d. Consider uploading reports for the commit 7eb934d to get more accurate results

Additional details and impacted files

@@               Coverage Diff                @@
##           branch-22.12   #11998      +/-   ##
================================================
+ Coverage         87.40%   88.12%   +0.71%     
================================================
  Files               133      133              
  Lines             21833    21991     +158     
================================================
+ Hits              19084    19380     +296     
+ Misses             2749     2611     -138     

Impacted Files	Coverage Δ
python/strings_udf/strings_udf/__init__.py	84.31% <0.00%> (-12.57%)	⬇️
python/cudf/cudf/io/text.py	91.66% <0.00%> (-8.34%)	⬇️
python/cudf/cudf/core/_base_index.py	81.28% <0.00%> (-4.27%)	⬇️
python/strings_udf/strings_udf/_typing.py	94.73% <0.00%> (-1.06%)	⬇️
python/cudf/cudf/testing/dataset_generator.py	72.83% <0.00%> (-0.42%)	⬇️
python/dask_cudf/dask_cudf/backends.py	84.90% <0.00%> (-0.37%)	⬇️
python/dask_cudf/dask_cudf/core.py	73.92% <0.00%> (-0.21%)	⬇️
python/cudf/cudf/io/orc.py	92.94% <0.00%> (-0.09%)	⬇️
python/cudf/cudf/core/dataframe.py	93.74% <0.00%> (-0.03%)	⬇️
python/cudf/cudf/__init__.py	90.69% <0.00%> (ø)
... and 23 more

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[eriknw]

CI passes! That's nice.

The lines I am proposing to remove originated in #1622 by @shwina (this commit specifically: 04c4fc6). @shwina, any idea why these lines were added and what they're supposed to do?

Recall that I encountered this bug when trying to do joins or merges by index on two dask_cudf dataframes, which use repartition, which uses loc. This affects many "normal" joins in my tests (on tiny data). In other words, I think this is important to fix.

[wence-]

I think we can delete some more code here...

[shwina]

@shwina, any idea why these lines were added and what they're supposed to do?

Alas, I have not. Happy to see code go away though!

[wence-]

Thanks!

[galipremsagar]

@gpucibot merge