Delete update branch when syncing a fork

[fthomas]

Closes #741.

[codecov]

Codecov Report

Merging #2371 (279e0c9) into master (5cf6463) will increase coverage by 0.10%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #2371      +/-   ##
==========================================
+ Coverage   80.36%   80.46%   +0.10%     
==========================================
  Files         139      139              
  Lines        2388     2396       +8     
  Branches       55       54       -1     
==========================================
+ Hits         1919     1928       +9     
+ Misses        469      468       -1     

Impacted Files	Coverage Δ
...n/scala/org/scalasteward/core/git/FileGitAlg.scala	98.33% <100.00%> (+0.02%)	⬆️
...in/scala/org/scalasteward/core/git/GenGitAlg.scala	92.85% <100.00%> (+0.26%)	⬆️
...main/scala/org/scalasteward/core/git/package.scala	100.00% <100.00%> (ø)
...n/scala/org/scalasteward/core/vcs/VCSRepoAlg.scala	81.81% <100.00%> (+5.34%)	⬆️
.../main/scala/org/scalasteward/core/git/Branch.scala	100.00% <0.00%> (+25.00%)	⬆️

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