range-diff: clear `other_arg` at end of function

range-diff: clear `other_arg` at end of function

We were leaking memory by not clearing `other_arg` after we were done
using it. Clear it after we've finished using it.

Note that this isn't strictly necessary since the memory will be
reclaimed once the command exits. However, since we are releasing the
strbufs, we should also clear `other_arg` for consistency.

Signed-off-by: Denton Liu <liu.denton@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>