grep: move thread initialization a little lower

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>