Incorporate CommitService.GetTreeEntries Gitaly call

What does this MR do?

Incoroporate CommitService.GetTreeEntries Gitaly call

Closes https://gitlab.com/gitlab-org/gitaly/issues/372

Are there points in the code the reviewer needs to double check?

No

Why was this MR needed?

Client implementation of https://gitlab.com/gitlab-org/gitaly/issues/320

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 https://gitlab.com/gitlab-org/gitaly/issues/372