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>

diff-lib.c

@@ -383,7 +383,7 @@ static inline void skip_same_name(struct cache_entry *ce, struct unpack_trees_op
  * For diffing, the index is more important, and we only have a
  * single tree.
  *
- * We're supposed to return how many index entries we want to skip.
+ * We're supposed to advance o->pos to skip what we have already processed.
  *
  * This wrapper makes it all more readable, and takes care of all
  * the fairly complex unpack_trees() semantic requirements, including