Fix update-deps with certain forms of the {tag, ...} type

Sometimes tags like 1.1-3-g3af5478 or d20b53f0 are encountered. The
first is the output of 'git describe', and the second is just a regular
git SHA. git fetch --tags will not pull these down, so do both a git
fetch and a git fetch --tags to get both SHAs and tags.

This is a little hard to test if you don't have a stale dep hanging around somewhere, because I don't know of a way to make a git checkout 'forget' SHAs once it has seen them, but I had a stale riak tree hanging around and was able to check this worked.