diff --no-index: don't leak buffers in queue_diff

diff --no-index: don't leak buffers in queue_diff

queue_diff uses two strbufs, and at the end of the function
strbuf_reset was called.  This only reset the length of the buffer -
any allocated memory was leaked.  Using strbuf_release fixes this.

Signed-off-by: Bobby Powers <bobbypowers@gmail.com>
Reviewed-by: René Scharfe <rene.scharfe@lsrfire.ath.cx>
Signed-off-by: Junio C Hamano <gitster@pobox.com>