Compute PR diff-ranges during init #6160

Job	Run time
Ruby analysis (macos-latest, nightly-latest)	16s
Ruby analysis (macos-latest, linked)	1m 15s
Ruby analysis (ubuntu-latest, linked)	51s
Ruby analysis (ubuntu-latest, nightly-latest)	41s
Ruby analysis (macos-latest, default)	1m 18s
Ruby analysis (ubuntu-latest, default)	49s
	5m 10s

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