Only resolve branch names to branches

Summary: Fixes T7100. In the bizarre case that a Git repository has a branch and tag with the same name, don't resolve branch names into tag names.

Test Plan: Test repo with branch and tag both named "git" no longer reports ambiguity.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T7100

Differential Revision: https://secure.phabricator.com/D12553