Enable verbose logging of git2go subcommands
Currently, git2go subcommands contain very complex logic for various tasks (e.g. squashing commits/resolving conflicts/commiting multiple changes). The subcommands lack observability since STDERR is used to return error messages. An alternative file descriptor should be used to allow git2go commands to relay back fine grain logs. When a git2go command exits with non-zero, or the logging level is set to a certain level, these fine grain logs should be emitted.