Improve ref resolution for Git branches and tags

Summary:
Fixes T7982.

• When resolving branches, make sure they get type 'branch'.
• Correctly resolve refs when a repository has a branch and tag with the same name.

Test Plan: Disabled ref cache and resolved refs in a Git repository with a 'master' tag and a 'master' branch. Saw refs resolve accurately.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T7982

Differential Revision: https://secure.phabricator.com/D12609