strbuf_addbuf(): allow passing the same buf to dst and src

If sb and sb2 are the same (i.e. doubling the string), the underlying
strbuf_add() can make sb2->buf invalid by calling strbuf_grow(sb) at
the beginning; if realloc(3) done by strbuf_grow() needs to move the
string, strbuf_add() will read from an already freed buffer.

Signed-off-by: Junio C Hamano <gitster@pobox.com>