diff-lib.c: fix misleading comments on oneway_diff()

diff-lib.c: fix misleading comments on oneway_diff()

20a16eb (unpack_trees(): fix diff-index regression., 2008-03-10) adjusted
diff-index to the new world order since 34110cd (Make 'unpack_trees()'
have a separate source and destination index, 2008-03-06).  Callbacks are
expected to return anything non-negative as "success", and instead of
reporting how many index entries they have processed, they are expected to
advance o->pos themselves.  The code did so, but a stale comment was left
behind.

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