This moves the call to setup_git_directory() for running "grep" from
the "git" wrapper to the implementation of the "grep" subcommand. A
new variable "use_index" is always true at this stage in the series,
and when it is on, we require that we are in a directory that is under
git control. To make sure we die the same way, we make a second call
into setup_git_directory() when we detect this situation.