Fix tag content display in Git

Summary: Fixes the junk I broke in D7484. Before that, tag content was a side effect of resolving the ref name. Now, fetch it explicitly in `diffusion.tagsquery`.

Test Plan: Looked at a tag, saw the annotation/message.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Differential Revision: https://secure.phabricator.com/D7485