avoid displaying the scanning annexed files message when repo is not large

avoid displaying the scanning annexed files message when repo is not large

Avoids users thinking this scan is a big deal, when it's not in the
majority of repos.

showSideActionAfter has some ugly caveats, since it has to display in
the background of another action. I could not see a better way to do it
and it works fine in this particular case. It also doesn't really belong
in Annex.Concurrent, but cannot go in Messages due to an import loop.

Sponsored-by: Dartmouth College's Datalad project