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