support for git tags that point to arbitrary git objects, instead of revisions

There is at least one known case of this in linux.git, with a tag that points to a tree (= the latest snapshot of linux source code before the migration to git).

Note: even though this is very Git-specific, and probably doesn't fit well in our general DB model, we do want to be able to round-trip on linux.git, because people are gonna try to do that.

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Migrated from T68 (view on Phabricator)