Repository branch count counts remote branches

Customer noticed that the branch count was inconsistent with the number of available branches in the dropdown for project mirrors. For example:

Here's how to reproduce:

1. Create a new project, import it from a URL, and click "Mirror project".
2. Push a new branch to the local project.
3. Observe the counts.

This appears to happen because Repository#branch_names filters by local branches but Repository#branch_count does not.

Zendesk: https://gitlab.zendesk.com/agent/tickets/81519