Move git.Diff to gitserver.Client

Created by: ryanslade

I needed to copy some of the mocking logic over from vcs/git. I tried to work around that for a while but it's pretty entangled with a bunch of other parts of codebase so I think we're better off making one change at a time. We can sort out mocking once everything is moved over to the client.

Closes https://github.com/sourcegraph/sourcegraph/issues/33313