gitaly, gitlab-ce, workhorse, (possibly) gitlab-shell: deprecate Repository.path

Wait for 9.2 with this. Needs #150 (closed)

Once everything and everyone addresses repositories with shard+relative_path we should remove code that supports Repository.path and make it an error in helper.GetRepoPath if Repository.path is set. Put a comment in the .proto to show that Repository.path is deprecated.

• put a deprecation comment // DEPRECATED: <ISSUE-LINK> in gitaly-proto gitaly-proto!22 (merged)
• find all client-side uses of RPC and list below
  • All of them :)
• update all client-side uses to no longer use RPC ADD Merge Request LINKS
• wait for a GitLab release in which the RPC is no longer occuring in client side code LINK TO GITLAB-CE RELEASE TAG
• delete the server side implementation of the old RPC in https://gitlab.com/gitlab-org/gitaly !179 (merged)