strbuf_split: add a max parameter

strbuf_split: add a max parameter

Sometimes when splitting, you only want a limited number of
fields, and for the final field to contain "everything
else", even if it includes the delimiter.

This patch introduces strbuf_split_max, which provides a
"max number of fields" parameter; it behaves similarly to
perl's "split" with a 3rd field.

The existing 2-argument form of strbuf_split is retained for
compatibility and ease-of-use.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>