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: NBrandon Casey <drafnel@gmail.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

strbuf.c

@@ -357,7 +357,6 @@ int strbuf_getwholeline(struct strbuf *sb, FILE *fp, int term)
 {
 	int ch;
 
-	strbuf_grow(sb, 0);
 	if (feof(fp))
 		return EOF;