[RFC] UI for clean-up of redundant tags

[Ian Clatworthy]

For reasons unknown, we occasionally end up with tags that
don't reference a revision known in the branch. I'll like
to clean these up and wanted input on how best to do that.

At least 3 options exist:

1. custom script just for me

2. add redundant tag checking to 'bzr check' and redundant
   tag removal to 'bzr reconcile'

3. Add --clean option to bzr tags. Maybe -- clean --dry-run
   as well to just see what --clean will do.

3 is my preferred option given that tag clean-up will take a
few seconds while check/reconcile take hours for branches
with deep history.

Thoughts?

Ian C.