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 a full git
fetch instead.
Merge pull request #154 from Vagabond/adt-update-tags

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