Entirely possible to tag something other than the revision which was built in SVN

Hiya, we're using ShipIt for our internal builds at my work, and I've found that we could sometimes have problems with the tags created not reflecting what was actually built as a debian package.

I've made some changes to store a little additional state and so tag a specific revision which fixes this in my branch at bobtfish/shipit@880d200802f0b19083a3
