patchfast-import: exit with proper message if not a git dir

> git fast-import expects to be run from an existing (possibly
> empty) repository.  It was dying with a suboptimal message if that
> wasn't the case.
> 
> Signed-off-by: Jean-Luc Herren <jlh@gmx.ch>
> ---
> fast-import.c |    1 +
> 1 files changed, 1 insertions(+), 0 deletions(-)
> 
> diff --git a/fast-import.c b/fast-import.c
> index 0d3449f..7f197d5 100644
> --- a/fast-import.c
> +++ b/fast-import.c
> @@ -2377,6 +2377,7 @@ int main(int argc, const char **argv)
> {
> 	unsigned int i, show_stats = 1;
> 
> +	setup_git_directory();
> 	git_config(git_pack_config);
> 	if (!pack_compression_seen && core_compression_seen)
> 		pack_compression_level = core_compression_level;