rebuilding from old cvs tags

[Bill Nottingham]

Mike Bonnet (mikeb at redhat.com) said: 
> I think immutable tags are the answer here.  We already kind of assume
> tags don't change once they're built, we might as well enforce it.  I
> know force-tag is convenient, but how much harder is it really to bump
> the revision number instead?

In the case of transient build errors, it's rather annoying.

Bill