combine-diff.c: type sanity

combine-diff.c: type sanity

- combine_diff() took cnt (count) which is unsigned in nature but the
  parameter type was declared as "int";
- find_next() took "uninteresting" parameter, which masked a static
  function of the same name;
- show_parent_lno() took an unused parameter "cnt";
- show_patch_diff() used a local variable in nested inner scope with
  the same name with different type, masking the one in the outer scope;
- the last loop in show_patch_diff iterated over lines so it should use
  the local variable "lno"

Signed-off-by: Junio C Hamano <junkio@cox.net>