Read RepoConfig only once when updating a repo

[fthomas]

This is just an optimization to not repeatedly reading and parsing the
repository config when working on a repo.

[codecov]

Codecov Report

Merging #522 into master will decrease coverage by 0.05%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master     #522      +/-   ##
==========================================
- Coverage   55.23%   55.17%   -0.06%     
==========================================
  Files          64       64              
  Lines         793      792       -1     
  Branches       40       38       -2     
==========================================
- Hits          438      437       -1     
  Misses        355      355

Impacted Files	Coverage Δ
...scala/org/scalasteward/core/update/FilterAlg.scala	60.6% <ø> (-1.16%)	⬇️
...ala/org/scalasteward/core/nurture/NurtureAlg.scala	0% <0%> (ø)	⬆️

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