Only link symbols if there might be any
fixes T8260. Only turn on symbol links if:
- The repository has any configuration about symbols, or
- There actually are symbols in the repository.
Test Plan: Look at revisions and files in various states of configurations and having symbols.
Reviewers: Blessed Reviewers, epriestley
Reviewed By: Blessed Reviewers, epriestley
Subscribers: joshuaspence, Korvin, epriestley
Maniphest Tasks: T8260
Differential Revision: https://secure.phabricator.com/D12946