strbuf: add fixed-length version of add_wrapped_text

strbuf: add fixed-length version of add_wrapped_text

The function strbuf_add_wrapped_text takes a NUL-terminated
string. This makes it annoying to wrap strings we have as a
pointer and a length.

Refactoring strbuf_add_wrapped_text and all of its
sub-functions to handle fixed-length strings turned out to
be really ugly. So this implementation is lame; it just
strdups the text and operates on the NUL-terminated version.
This should be fine as the strings we are wrapping are
generally pretty short.  If it becomes a problem, we can
optimize later.

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