Three recent todos have needed a way to introspect the matchers built from
preferred content expressions and some command-line options, to determine
what information they use. So implemented that today.

With that, it was possible to double the seeking speed of 
`git-annex sync --all` when include=/exclude= are not used
in preferred content. And the seeking speed of commands like
`git-annex find --copies=2` and `--in remote` improved by around 20%.

Today's work was sponsored by Jake Vosloo
[on Patreon](https://www.patreon.com/joeyh).
