Adding support for enabling auto merge on repositories

[Richard-de-Vries]

Hey!

I'm using the github terraform plugin which makes use of this api. I would like to add support for the enabling of automatic merges on a repository level. integrations/terraform-provider-github#649
I'm new to open source but i'm willing to make a PR.

[gmlewis]

I'm new to open source but i'm willing to make a PR.

Sounds great, @Richard-de-Vries ! Thank you.

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