Bug 405970

Cause: QN-UnQ

• Evidence: "So, for a long time, we've been re-deriving all regexp groups on every run of checksetup.pl when there was no need to. Bugzilla::Group itself does the rederiving correctly when you create a new group, and editgroups.cgi does the rederiving when you change the regex. When you update somebody's login name, their regexp-based memberships are also automatically rederived."
• Evidence: " I fixed it the right way--now we actually only rederive regex groups when there exist userregexp groups and there exist no users with regex-derived group memberships"