Handle tag tags properly in discovery

Summary:
Fixes T11180. In Git, it's possible to tag a tag (????). When you do, we try to log the tag-object, which automatically resolves to the commit and fails.

Just skip these. If "A" points at "B" which points at "C", it's fine to ignore "A" and "B" since we'll get the same stuff when we process "C".

Test Plan:

• Tagged a tag.
• Pushed it.
• Discovered it.
• Before patch: got exception similar to the one in T11180.
• After patch: got tag-tag skipped. Also got slightly better error messages.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T11180

Differential Revision: https://secure.phabricator.com/D16149