Fixed issue2550572: setting nosy=+foo on multiple issues gives them all the same exact nosy list. Added a missing reinitialization that has to occur every time though the loop in do_set. Manually tested with: python roundup/scripts/roundup_admin.py -i demo set issue184,issue17 nosy=demo python roundup/scripts/roundup_admin.py -i demo set issue17 nosy=+alpha,+anonymous python roundup/scripts/roundup_admin.py -i demo set issue184 nosy=+beta,+anonymous python roundup/scripts/roundup_admin.py -i demo set issue184,issue17 nosy=-demo,-anonymous,+admin issue17 nosy was admin,alpha issue 184 nosy was admin,beta after tests.

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