Merge 'git_tag' functionality into 'git' source

This branch merges functionality added to the forked 'git_tag' source back into 'git'.

The reason for the fork was that some desired functionality was rejected from being added into the 'git' source (see #220 (closed)). I re-raised this in a mailing list post on the topic -- the response indicates to me that the issue no longer exists.

The main additions are:

1. The source will fetch only the given ref when it is an exact tag (-0-g)
2. The source can be configured to choose the latest tagged commit when tracking with a new option 'track-latest-tag'. (This is called 'track-tags' in 'git_tag')
3. 'track' may now be a list; the source chooses the overall latest commit/tag found on any when tracking. (This was implemented by 'track-extra' in 'git_tag')