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

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 <redacted>
Signed-off-by: Junio C Hamano <redacted>