A Git Tag object. The tag object is very much like a Commit.t object - it contains a tagger, a date, a message, and a pointer. Generally, the tag points to a commit rather than a tree. It's like a branch reference, but it never moves - it always points to the same commit but gives it a friendlier name.

Functor building an implementation of the tag structure. The functor returns a structure containing a type hash of digests and a type t of tags (structurally equal to t).