$ git for-each-ref --format='%(refname) %(objectname), %(objecttype), %(*objecttype)'
refs/heads/master 5230675b77c5d67380c2a8aba715398c5ee1db9c, commit,
refs/tags/annotate 1043ce6092f54480a22223752823aafa8e548d7c, tag, commit
refs/tags/tag-of-file d077588c31c154f2544980bd4fd37ece0136020b, blob,

I couldn't tag a tag though.

I would expect the $ref_type check to cause us to incorrectly skip checking normal tags (of commits), but we do want to discover commits that are reachable from only tags. Am I misunderstanding that?

I intended to say that "If objecttype is a commit, or if objecttype is tag that points to (*objecttype) a commit, then handle it".

Then I flipped all the conditions.

I'm going to update the GT unit test to include all these edges.

In the new GT.git.tgz:

$ git for-each-ref
763d4ab372445551c95fb5cccd1a7a223f5b2ac8 commit refs/heads/master
41fa35914aa19c1aa6e57004d9745c05929c3563 commit refs/tags/plain-tag
270e6739dc7e08a5f2f1fdd7bf188ca64eaeda32 blob   refs/tags/tag-a-blob

41fa35914aa19c1aa6e57004d9745c05929c3563 is a commit that is only reachable from the plan tag; It's parent is 763d4ab372445551c95fb5cccd1a7a223f5b2ac8, which is master.