Add flag to decide which (local|remote) git command to use

Created by: sashaostrikov

There are 2 commits:

1. A little refactoring which moves GitCommand interface and both implementations to a separate file. Intention is that client.go had almost 1800 lines and moving some code can benefit reading. Also I believe that this will separate two different things: client creates a command, but don't really owns it.
2. Introducing of the flag itself

This PR goes on top of the https://github.com/sourcegraph/sourcegraph/pull/34341 and will be merged after the first one is merged Thanks Ryan :)

Test plan

Existing tests pass