strbuf: allow strbuf_split to work on non-strbufs

strbuf: allow strbuf_split to work on non-strbufs

The strbuf_split function takes a strbuf as input, and
outputs a list of strbufs. However, there is no reason that
the input has to be a strbuf, and not an arbitrary buffer.

This patch adds strbuf_split_buf for a length-delimited
buffer, and strbuf_split_str for NUL-terminated strings.

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