Currently running `git annex untrust` on a remote that is already untrusted
will still update the git-annex branch. And that should not change, because
one use case is overriding a change made elsewhere, in a semi-split brain
situation, and so indication it's still untrusted is useful.

But, if a program, or even an external special remote wants to untrust
itself, it currently needs to parse `git annex info` to see if it's
currently untrusted, and only run untrust if not, otherwise each time it
runs it will pile up a change in the git-annex branch.

So, suggest a flag to avoid generally unnecessary branch update.
It would also make sense to add it to the other trust level commands,
and possibly other config setting commands. --[[Joey]]

[[!tag needsthought]]
