Add AllowUpdateBranch to the Repository struct

[MarkRosemaker]

I noticed that I can edit all the settings for a repository via the API except the setting:

Always suggest updating pull request branches
Whenever there are new changes available in the base branch, present an “update branch” option in the pull request.

This PR aims to change that. I verified the changed code by making an edit request to one of my repositories.

This PR will solve #2312.

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[gmlewis]

Thank you, @MarkRosemaker !
Instead of editing the auto-generated files manually, please run go generate ./... and push (not force-push) the commit to this PR. This should fix the broken tests. Please see CONTRIBUTING.md for more information.

[MarkRosemaker]

Thank you, @MarkRosemaker ! Instead of editing the auto-generated files manually, please run go generate ./... and push (not force-push) the commit to this PR. This should fix the broken tests. Please see CONTRIBUTING.md for more information.

Thank you for the suggestion, @gmlewis ! Done. :)

[codecov]

Codecov Report

Merging #2313 (ecb6ec9) into master (4d609bd) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #2313   +/-   ##
=======================================
  Coverage   97.84%   97.84%           
=======================================
  Files         115      115           
  Lines       10415    10416    +1     
=======================================
+ Hits        10191    10192    +1     
  Misses        156      156           
  Partials       68       68           

Impacted Files	Coverage Δ
github/repos.go	98.70% <100.00%> (+<0.01%)	⬆️

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[gmlewis]

Thank you, @MarkRosemaker !
LGTM.
Merging.