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.
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