Use Gitaly for getting Branch/Tag counts

Conversation: gitaly#214 (closed)

What does this MR do?

Replace current rugged-code with Gitaly for getting branch/tag-counts.

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

closes gitaly#157 (closed)