Line changes can be negative, and must parse from extra heartbeats json too

[alanhamlett]

When line changes are negative, it means lines were deleted.

[codecov]

Codecov Report

❌ Patch coverage is 55.88235% with 15 lines in your changes missing coverage. Please review.
✅ Project coverage is 63.00%. Comparing base (7f38194) to head (472b48d).
⚠️ Report is 2 commits behind head on develop.

Files with missing lines	Patch %	Lines
pkg/params/params.go	55.88%	9 Missing and 6 partials ⚠️

@@             Coverage Diff             @@
##           develop    #1219      +/-   ##
===========================================
- Coverage    63.05%   63.00%   -0.06%     
===========================================
  Files          393      393              
  Lines        17073    17073              
===========================================
- Hits         10765    10756       -9     
- Misses        5766     5772       +6     
- Partials       542      545       +3     

Flag	Coverage Δ
unittests	63.00% <55.88%> (-0.06%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
pkg/params/params.go	79.73% <55.88%> (-1.08%)	⬇️

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