Add git-tag.py plugin

In freedesktop-sdk we have wanted a way to track the latest tag on a git branch for a long time. This [1] BuildStream branch implemented the changes necessary, but it looks unlikely to ever be merged, see the discussion here [2]. As a result I have added the functionality into a plugin we can use instead of the BuildStream git plugin.

I had to use importlib to access the git plugin, but doing this makes this plugin far easier to maintain, as only 3 methods will actually require maintenance: GitMirror.latest_commit(), GitSource.configure() and GitSource.track(), the rest are inherited from the BuildStream plugin.

• [1] buildstream!303 (closed)
• [2] buildstream#220 (closed)