Extending of git.Repository with Remote interface

In order to support migration of functionality from Ruby to Go implementation it makes sense to extend existing git.Repository interface with new methods in order to re-use them during code porting. This change adds partial implementation of the git remote sub-command. It is declared as a separate interfaces and could be used separately from the Repository interface as well. Each command accepts a configuration struct that allows to set optional flags, etc. Currently it is used only in tests, but would be in use in the future to omit code duplication.

Part of: #1466 (closed)