getDiffRanges: better fallback for absent patch #3570

Job	Run time
Diagnostic export (ubuntu-latest, linked)	56s
Diagnostic export (macos-latest, nightly-latest)	1m 24s
Diagnostic export (windows-latest, nightly-latest)	3m 54s
Diagnostic export (windows-latest, linked)	2m 38s
Diagnostic export (ubuntu-latest, nightly-latest)	58s
Diagnostic export (macos-latest, linked)	1m 32s
	11m 22s

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