Add the ability to modify whether automerging is enabled for a repository via the REST API

It would be useful to query whether automerging has been enabled for a repository from the REST API. It would also be useful to be able to toggle it on and off remotely. The branch model REST endpoint doesn't include whether automerging is enabled which is where I would expect it to be based on the location of the setting in the UI.

The reason this would be useful for me is that I am writing a custom plugin that changes its functionality based on whether automerging is enabled or not. I'd like to write integration tests that can remotely toggle the automerging setting on on and off to adaquately cover the code paths in my plugin.