Simplify and fix runahead computation. #6696

Job	Run time
1/4	13m 11s
2/4	14m 43s
3/4	14m 32s
4/4	13m 39s
py-3.11	13m 57s
flaky	6m 6s
_remote_background_indep_poll	5m 30s
_remote_background_indep_tcp	7m 4s
macos 1/5	37m 36s
macos 2/5	45m 2s
codecov	0s
	2h 51m 20s

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