git_tag: Use any tag, add default behaviour as fallback

Tom Coldrickrequested to merge
coldtom/git_tag into master

When tracking a lot of git_tag elements, occasionally there may be a repository without annotated tags or any tags at all, this should not break bst track. Therefore this adds changes the track-tags behaviour to look for any tag, and if this fails to default to the standard behaviour of track on git elements - track the latest HEAD

Closes #21 (closed)

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