getFileDiffsWithBasehead(): use CODE_SCANNING_REPOSITORY if present #3982

Job	Run time
Diagnostic export (windows-latest, linked)	3m 23s
Diagnostic export (ubuntu-latest, nightly-latest)	1m 3s
Diagnostic export (windows-latest, nightly-latest)	2m 44s
Diagnostic export (ubuntu-latest, linked)	1m 5s
Diagnostic export (macos-latest, linked)	1m 3s
Diagnostic export (macos-latest, nightly-latest)	1m 1s
	10m 19s

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