Behavior change: git-annex trust now needs --force

Behavior change: git-annex trust now needs --force

Since unconsidered use of trusted repositories can lead to data loss.

Trusted has always been this way, but it used to be acceptable for
git-annex to be set up so that data could be lost without using --force,
and most or all other ways that can happen have already been eliminated.

This commit was sponsored by Mark Reidenbach on Patreon.