git: Move ShortLog to gitserver client
Created by: ryanslade
Move vcs/git.ShortLog
to gitserver.Client.ShortLog
.
This is an example PR to get thoughts on this refactoring pattern and is based on this doc.
If @sourcegraph/repo-management are happy with this approach then the plan is to move all
other methods in vcs/git
onto the client and I'll create another issue to track that.
Test plan
Code only moved around, all tests still pas