Fix forking an empty repository

[segevfiner]

Forking an empty repo didn't copy the IsEmpty flag which lead to Git errors, and once we did copy it, the repoAssignment middleware didn't populate BaseRepo which lead to a nil error from the templates.

Fixes #6633 (Assuming there aren't issues in other places)

cc @shay-k

[codecov-io]

Codecov Report

Merging #6637 into master will increase coverage by <.01%.
The diff coverage is 50%.

@@            Coverage Diff             @@
##           master    #6637      +/-   ##
==========================================
+ Coverage   40.53%   40.53%   +<.01%     
==========================================
  Files         406      406              
  Lines       54492    54494       +2     
==========================================
+ Hits        22086    22088       +2     
  Misses      29370    29370              
  Partials     3036     3036

Impacted Files	Coverage Δ
models/repo.go	46.95% <100%> (+0.03%)	⬆️
modules/context/repo.go	58.48% <40%> (+0.08%)	⬆️

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[lunny]

This should be back port to v1.8