Use git abstract class for git_tag

As git_tag only changes a few behaviours of BuildStream's git plugin, it will be far easier to maintain if we properly subclass from the abstract git source.

Also, in order to be consistent with the git plugin in BuildStream we need to change the track-tags attribute, as there is a different feature using the same parameter. This also makes me question the naming of the plugin in general, but it's probably not worth a change that drastic.