git source plugin: Track git tags and save them to reproduce a minimum shallow repository

Instead of tag information being fetched which can change with time,
they are tracked and saved in the projects.refs/.bst. Then we re-tag
automatically the closest tag so that `git describe` works and is
reproducible.

This new feature is opt-in with the new `track-tags` configuration,
and must be used to fix modules which are broken by our new policy
of omitting the `.git/` repository when staging git sources.

This fixes issue #487