Fix JENKINS-46046

[tilln]

Proper handling of existing reports for relative comparisons (JENKINS-46046)

[codecov-io]

Codecov Report

Merging #127 into master will increase coverage by 3.25%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #127      +/-   ##
==========================================
+ Coverage   64.26%   67.52%   +3.25%     
==========================================
  Files          44       44              
  Lines        3663     3667       +4     
  Branches      625      625              
==========================================
+ Hits         2354     2476     +122     
+ Misses       1132      986     -146     
- Partials      177      205      +28

Impacted Files	Coverage Δ
...dson/plugins/performance/PerformancePublisher.java	85.59% <100%> (+19.14%)	⬆️
...ns/performance/actions/PerformanceBuildAction.java	81.25% <0%> (+3.12%)	⬆️

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[undera]

Thanks for your contribution.
Having unit test checking this feature would help to keep it working in the future.