lib/model: Don't set ignore bit when it's already set

This adds a metric for "committed items" to the database instance that I
use in the test code, and a couple of tests that ensure that scans that
don't change anything also don't commit anything.

There was a case in the scanner where we set the invalid bit on files
that are ignored, even though they were already ignored and had the
invalid bit set. I had assumed this would result in an extra database
commit, but it was in fact filtered out by the Set... Anyway, I think we
can save some work on not pushing that change to the Set at all.

GitHub-Pull-Request: https://github.com/syncthing/syncthing/pull/3298
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