Simplify and fix runahead computation. #6745

Job	Run time
1/4	14m 36s
2/4	17m 39s
3/4	14m 33s
4/4	13m 59s
py-3.11	13m 54s
flaky	6m 57s
_remote_background_indep_poll	5m 30s
_remote_background_indep_tcp	7m 2s
macos 1/5	21m 17s
macos 2/5	21m 8s
codecov	6s
	2h 16m 41s

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