Add a "CommitSymbolRef" for resolving symbolic commits into stable commit hashes

Summary:
Ref T13490. Frequently, we know the symbolic name of a commit (like "master") but need the immutable identifier for it (the commit hash).

Provide a Ref and Query for doing this lookup.

Test Plan: Ran arc inspect symbol(...) with various symbols, saw appropriate resolutions, nulls, or errors.

Maniphest Tasks: T13490

Differential Revision: https://secure.phabricator.com/D21090