Add AllowUpdateBranch to the Repository definition

[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.

After some searching, if found that even though it's not documented yet, there is an allow_update_branch field in the REST Repo API. (h/t to @pmjacinto for the suggestion.)

I verified this by adding the field to my fork and making an edit request to one of my repositories.
Please see: #2313