[[forum/refresh_and_setup]] indicates some confusion between --setup
and -setup. Both work, but it's clearer if we stick to one in
documentation and code.
A 2012 commit to [[plugins/theme]] claims that "-setup" is required
and "--setup" won't work, but I cannot find any evidence in ikiwiki's
source code that this has ever been the case.
There are two sub-caces. If both source files still exist, the winner that
renders the destination file is undefined. If one source file is deleted
and the other added, in a refresh, the new file will take over the
destination file.