Persist update branch in PullRequestRepository

[fthomas]

The goal here is to use the stored update branches instead of calling
branchFor for obsolete pull requests. It should allow us to change
branchFor in the future without breaking the closing of obsolete PRs.

[codecov]

Codecov Report

Merging #2307 (054b6a6) into master (14d66dc) will decrease coverage by 0.30%.
The diff coverage is 42.85%.

@@            Coverage Diff             @@
##           master    #2307      +/-   ##
==========================================
- Coverage   78.47%   78.17%   -0.31%     
==========================================
  Files         136      136              
  Lines        2290     2318      +28     
  Branches       55       62       +7     
==========================================
+ Hits         1797     1812      +15     
- Misses        493      506      +13     

Impacted Files	Coverage Δ
...ala/org/scalasteward/core/nurture/NurtureAlg.scala	0.00% <0.00%> (ø)
...cala/org/scalasteward/core/update/PruningAlg.scala	90.14% <0.00%> (-1.29%)	⬇️
...rg/scalasteward/core/vcs/data/PullRequestOut.scala	100.00% <ø> (+33.33%)	⬆️
.../scalasteward/core/vcs/data/PullRequestState.scala	72.72% <0.00%> (-16.17%)	⬇️
...lasteward/core/nurture/PullRequestRepository.scala	100.00% <100.00%> (ø)

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