strbuf.c: remove unnecessary strbuf_grow() from strbuf_getwholeline()

This use of strbuf_grow() is a historical artifact that was once used to ensure that strbuf.buf was allocated and properly nul-terminated. This was added before the introduction of the slopbuf in b315c5c0, which guarantees that strbuf.buf always points to a usable nul-terminated string. So let's remove it. Signed-off-by: Brandon Casey <drafnel@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>