Try to refine pivot in choose_pivot

[tkf]

[codecov[bot]]

Codecov Report

Merging #188 (15e9ee1) into copy-twice (5dd6143) will increase coverage by 0.46%. The diff coverage is 76.00%.

@@              Coverage Diff               @@
##           copy-twice     #188      +/-   ##
==============================================
+ Coverage       71.50%   71.97%   +0.46%     
==============================================
  Files              11       11              
  Lines             537      553      +16     
==============================================
+ Hits              384      398      +14     
- Misses            153      155       +2     

Impacted Files	Coverage Δ
src/quicksort.jl	65.67% <72.72%> (+2.11%)	:arrow_up:
src/utils.jl	65.71% <100.00%> (+1.42%)	:arrow_up:

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