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

[bobtfish]

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