diff: get rid of redundant 'dense' argument

diff: get rid of redundant 'dense' argument

Get rid of 'dense' argument that is redundant for every function that has
'struct rev_info *rev' argument as well, as the value of 'dense' passed is
always taken from 'rev->dense_combined_merges' field.

The only place where this was not the case is in 'submodule.c' where
'diff_tree_combined_merge()' was called with '1' for 'dense' argument. However,
at that call the 'revs' instance used is local to the function, and we now just
set 'revs->dense_combined_merges' to 1 in this local instance.

Signed-off-by: Sergey Organov <sorganov@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>