gitserver: Make Cmd fields private

Created by: ryanslade

This is in preparation for having two Command implementations, one that calls
out to gitserver over the network and one that runs against a local git directory. This will allow us to more easily test inside the gitserver package and should remove the need to run gitserver instances in tests. It may also allow us to remove a lot of the mocking code we use today.

All fields inside of gitserver.Cmd were made private. Methods were introduced where we need to read or write the values outside of the package.

In the case of the Repo field, we added it to the constructor as in all places it was used previously we create a Cmd and then set the value of the Repo field on the next line so this could be collapsed.

Test plan

Code only moved around. All tests still pass