Compute PR diff-ranges during init #6161

Job	Run time
Ruby analysis (ubuntu-latest, default)	54s
Ruby analysis (ubuntu-latest, linked)	47s
Ruby analysis (macos-latest, nightly-latest)	14s
Ruby analysis (ubuntu-latest, nightly-latest)	41s
Ruby analysis (macos-latest, default)	53s
Ruby analysis (macos-latest, linked)	1m 25s
	4m 54s

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