BUG: Incorrect value updating for groupby.cummin/max (#15635)

[mroeschke]

• closes BUG: cython groupby cummin/cummax assumes groups ordered #15635
• tests added / passed
• passes git diff upstream/master | flake8 --diff

Nice catch @adbull. The original implementation was incorrectly setting the incorrect cummin/max value.

[codecov-io]

Codecov Report

Merging #15642 into master will decrease coverage by 0.02%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master   #15642      +/-   ##
==========================================
- Coverage   91.02%      91%   -0.03%     
==========================================
  Files         143      143              
  Lines       49324    49324              
==========================================
- Hits        44898    44886      -12     
- Misses       4426     4438      +12

Impacted Files	Coverage Δ
pandas/io/gbq.py	25% <0%> (-58.34%)	❌
pandas/tools/merge.py	91.76% <0%> (-0.35%)	❌
pandas/core/common.py	90.96% <0%> (-0.34%)	❌
pandas/core/frame.py	97.87% <0%> (-0.1%)	❌

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[jreback]

thanks!