reprepro-import.sh: allow import of existing files #405

Job	Run time
unittests	11s
sanity	16s
focal, broken / build debs	2m 21s
focal, broken, cont-on-err / build debs	3m 59s
bookworm, good, verbose=bloom / build debs	3m 45s
jammy, good, verbose=true / build debs	3m 52s
jammy, good, timeout=3 / build debs	3m 8s
jammy push / deploy	17s
bookworm squash / deploy	10s
	17m 59s

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