Automerge does not work is merge commits are not allowed

[paymog]

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Overview of the Issue

Automerge does not work if merge commits are not allowed.

Reproduction Steps

1. Disable merge commits
2. Enable automerge
3. put up pr and plan/apply it
4. see failure

Logs

atlantis-0 atlantis {"level":"error","ts":"2024-08-09T15:04:25.498Z","caller":"vcs/instrumented_client.go:241","msg":"Unable to merge pull, error: merging pull request: PUT https://api.github.com/repos/goldsky-io/goldsky-infra/pulls/1302/merge: 405 This branch must not contain merge commits. You're not authorized to push to this branch. Visit https://docs.github.com/repositories/configuring-branches-and-merges-in-your-repository/managing-protected-branches/about-protected-branches for more informati []","json":{"repo":"goldsky-io/goldsky-infra","pull":"1302"},"stacktrace":"github.com/runatlantis/atlantis/server/events/vcs.(*InstrumentedClient).MergePull\n\tgithub.com/runatlantis/atlantis/server/events/vcs/instrumented_client.go:241\ngithub.com/runatlantis/atlantis/server/events/vcs.(*ClientProxy).MergePull\n\tgithub.com/runatlantis/atlantis/server/events/vcs/proxy.go:93\ngithub.com/runatlantis/atlantis/server/events.(*AutoMerger).automerge\n\tgithub.com/runatlantis/atlantis/server/events/automerger.go:35\ngithub.com/runatlantis/atlantis/server/events.(*ApplyCommandRunner).Run\n\tgithub.com/runatlantis/atlantis/server/events/apply_command_runner.go:184\ngithub.com/runatlantis/atlantis/server/events.(*DefaultCommandRunner).RunCommentCommand\n\tgithub.com/runatlantis/atlantis/server/events/command_runner.go:367"}
atlantis-0 atlantis {"level":"error","ts":"2024-08-09T15:04:25.498Z","caller":"events/automerger.go:38","msg":"automerging failed: merging pull request: PUT https://api.github.com/repos/goldsky-io/goldsky-infra/pulls/1302/merge: 405 This branch must not contain merge commits. You're not authorized to push to this branch. Visit https://docs.github.com/repositories/configuring-branches-and-merges-in-your-repository/managing-protected-branches/about-protected-branches for more informati []","json":{"repo":"goldsky-io/goldsky-infra","pull":"1302"},"stacktrace":"github.com/runatlantis/atlantis/server/events.(*AutoMerger).automerge\n\tgithub.com/runatlantis/atlantis/server/events/automerger.go:38\ngithub.com/runatlantis/atlantis/server/events.(*ApplyCommandRunner).Run\n\tgithub.com/runatlantis/atlantis/server/events/apply_command_runner.go:184\ngithub.com/runatlantis/atlantis/server/events.(*DefaultCommandRunner).RunCommentCommand\n\tgithub.com/runatlantis/atlantis/server/events/command_runner.go:367"}

[paymog]

It would be great if we could have a config option to choose between merge and rebase when merging the PR