Add support for merging results

[tonerdo]

Fixes #36

[codecov]

Codecov Report

Merging #186 into master will decrease coverage by 3.91%.
The diff coverage is 13.88%.

@@            Coverage Diff             @@
##           master     #186      +/-   ##
==========================================
- Coverage   98.93%   95.01%   -3.92%     
==========================================
  Files          15       15              
  Lines        1497     1564      +67     
==========================================
+ Hits         1481     1486       +5     
- Misses         16       78      +62

Impacted Files	Coverage Δ
src/coverlet.core/CoverageResult.cs	16.21% <0%> (-83.79%)	⬇️
src/coverlet.core/Coverage.cs	99.42% <100%> (+0.01%)	⬆️

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[Jenan]

Could you provide some details how to use this new feature? :)
Thanks

[tonerdo]

@Jenan

Merging Documentation:
https://github.com/tonerdo/coverlet#merging-results
https://github.com/tonerdo/coverlet#merging-results-1

[MortenChristiansen]

The merge option only works with the native json format. How would I merge results in the cobertura format?

[MortenChristiansen]

I got it working by outputting both the cobertura and json formats, though it required me to create a json coverage file containing {} first. Otherwise the first project in the solution fails as it has nothing to merge with.

[tonerdo]

Merging with non coverlet json formats might not ever be a supported option. It's way more easier with the coverlet json because it's just a serialization of the internal CoverageResult object. Other formats require more pre-processing and some information is lost in translation