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.