Migrate temp_repo.go to use git.NewCommand (#8918)
This PR migrates temp_repo.go to use git.NewCommand instead creating processes by itself - this fixes the problem underlying PR #8905.
There are other places that run git outside of the controlled locale defined in #8548 but temp_repo.go is the only cause of failure of local testing in cases where English is not the default - implying that error messages from those other commands are not interpreted.