Add a "RevisionSymbolRef", revision commit messages, and make "--explore" recursive

Summary:
Ref T13490.

• Support resolution of revision symbols into revision objects.
• Align commit inspection better against commit symbols.
• Make "arc inspect --explore" recursively load all object hardpoints.
• Add a "commit message" hardpoint to "RevisionRef".

Test Plan: Used "arc inspect" to resolve commits and revisions. Used "--explore" to see the whole tree.

Maniphest Tasks: T13490

Differential Revision: https://secure.phabricator.com/D21091