debug: add a debug::unbundle command that simulate the unbundle from a push The code have different behavior when the unbundle comes from a push, so we introduce a command that can simulate such unbundle. For our copy of mozilla-try-2023-03-22, this make the unbundle jump from 2.5 seconds (with `hg unbundle`) to 15 seconds (with `hg debug::unbundle`). That 15 seconds timings is consistent with the issue seen in production.