getDiffRanges: better fallback for absent patch #3569

Job	Run time
Diagnostic export (ubuntu-latest, linked)	1m 0s
Diagnostic export (windows-latest, linked)	3m 28s
Diagnostic export (ubuntu-latest, nightly-latest)	57s
Diagnostic export (windows-latest, nightly-latest)	5m 11s
Diagnostic export (macos-latest, nightly-latest)	1m 22s
Diagnostic export (macos-latest, linked)	1m 32s
	13m 30s

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