Conversation: Extend CommitService::CountCommits with extra options

Extends ~Conversation: #330 (closed)

• RPC Design: #413 (closed) gitaly-proto!47 (merged)

• Server Implementation: #414 (closed) !252 (merged)

• ~"Client Implementation": #415 (closed) https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/13121

• ~"Acceptance Testing": #517 (closed)

Gitlab::Git::Repository#count_commits

We can migrate Gitlab::Git::Repository#count_commits if we add three options to CommitService.CountCommits

message CountCommitsRequest {
  Repository repository = 1;
  bytes revision = 2;
  // Optional
  google.protobuf.Timestamp after = 2;
  // Optional
  google.protobuf.Timestamp before = 3;
  // Optional
  bytes path = 5;
}

These options translate into git rev-list options. While we're at it, we can change the Go implementation so that it uses git rev-list --count and we don't have to scan lines.§