update-index: drop unused argc from do_reupdate()

The parse-options callback for --again soaks up all remaining options by manipulating the parse_opt_ctx's argc and argv fields. Even though it has to look at both, the actual parsing happens via the do_reupdate() helper, which only looks at the argv half (by passing it along to parse_pathspec). So that helper doesn't need to see argc at all. Note that the helper does look at "argv + 1" without confirming that argc is greater than 0. We know this is correct because it is skipping past the actual "--again" string, which will always be present. However, to make what's going on more obvious, let's move that "+1" into the caller, which has the matching "-1" when fixing up the ctx's argc/argv. Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>