Tag tracking does not work for extra branches

The git_tag plugin now has a feature for tracking additional branches, using the track-extra parameter, however at present this is broken.

When we clone a git repository we do not checkout each of the branches we intend to track, so they are not present in the local repository for use with git describe. As a result only the branch specified in track will return a proper result. Thankfully this should be fixed by adding the remote name to the call of git describe.