Got unwanted content to be dropped from the local repo, as well as remotes
when doing the expensive scan. I kept the scan a single pass for now,
need to revisit that later to drop content before transferring more.
Also, when content is downloaded or uploaded, this can result in it
needing to be dropped from somewhere, and the assistant handles that too.

There are some edge cases with hypothetical, very weird preferred
content expressions, where the assistant won't drop content right away.
(But will later in the expensive scan.) Other than those, I think I have
nearly all content dropping sorted out. The only common case I know of where
unwanted content is not dropped by the assistant right away is when a file
is renamed (eg, put in a "Trash" directory).

In other words, repositories put into the transfer group will now work as
described, only retaining content as long as is needed to distribute it to
clients. Big milestone!
