Split getRepoConfig into readRepoConfig{,OrDefault}

[fthomas]

This will be useful for #592
where we only want to cache actual RepoConfigs and not also the default
values.

[codecov]

Codecov Report

Merging #650 into master will increase coverage by 0.04%.
The diff coverage is 85.71%.

@@            Coverage Diff            @@
##           master    #650      +/-   ##
=========================================
+ Coverage   60.15%   60.2%   +0.04%     
=========================================
  Files          66      66              
  Lines         876     877       +1     
  Branches       17      19       +2     
=========================================
+ Hits          527     528       +1     
  Misses        349     349

Impacted Files	Coverage Δ
...ala/org/scalasteward/core/nurture/NurtureAlg.scala	0% <0%> (ø)	⬆️
.../org/scalasteward/core/repoconfig/RepoConfig.scala	100% <100%> (ø)	⬆️
...g/scalasteward/core/repoconfig/RepoConfigAlg.scala	100% <100%> (ø)	⬆️

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