grep: move thread initialization a little lower

Originally, we set up the threads for grep before parsing the non-option arguments. In 53b8d931b (grep: disable threading in non-worktree case, 2011-12-12), the thread code got bumped lower in the function because it now needed to know whether we got any revision arguments. That put a big block of code in between the parsing of revs and the parsing of pathspecs, both of which share some loop variables. That makes it harder to read the code than the original, where the shared loops were right next to each other. Let's bump the thread initialization until after all of the parsing is done. Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>