getFileDiffsWithBasehead(): use CODE_SCANNING_REPOSITORY if present #3986

Job	Run time
Diagnostic export (ubuntu-latest, nightly-latest)	1m 6s
Diagnostic export (windows-latest, linked)	4m 51s
Diagnostic export (macos-latest, nightly-latest)	2m 8s
Diagnostic export (macos-latest, linked)	1m 2s
Diagnostic export (ubuntu-latest, linked)	1m 2s
Diagnostic export (windows-latest, nightly-latest)	5m 42s
	15m 51s

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