Add a configuration option for `merge --no-wait`

[danielgafni]

While I think --no-wait should be the default behavior (who wants to block their terminal until CI and other checks run - possibly a couple dozens of minutes - by default?), I believe there should be at least be a configuration option to opt-in into --no-wait.

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Edit: oh ok, I get why it might be the current default. This is because stax merge cannot trigger automerge. Therefore #64 should be resolved first.