fast-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: NJean-Luc Herren <jlh@gmx.ch>
Acked-by: NShawn O. Pearce <spearce@spearce.org>

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;