Simplify sortIssues implementation

[alexandear]

With the help of slices.SortFunc and the cmp package, this PR simplifies the implementation of sortIssues.

[codecov-commenter]

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Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 66.62%. Comparing base (1216c9b) to head (25c4ddc).
Report is 30 commits behind head on master.

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Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1277      +/-   ##
==========================================
- Coverage   68.49%   66.62%   -1.88%     
==========================================
  Files          75       75              
  Lines        4384     5186     +802     
==========================================
+ Hits         3003     3455     +452     
- Misses       1233     1582     +349     
- Partials      148      149       +1     

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