combine-diff: move formatting logic to show_combined_diff()

This way, diff-files can make use of it.  Also implement the
full suite of what diff_flush_raw() supports just for
consistency.  With this, 'diff-tree -c -r --name-status' would
show what is expected.

There is no way to get the historical output (useful for
debugging and low-level Plumbing work) anymore, so tentatively
it makes '-m' to mean "do not combine and show individual diffs
with parents".

diff-files matches diff-tree to produce raw output for -c.  For
textual combined diff, use -p -c.

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

diff-files.c

@@ -88,9 +88,8 @@ int main(int argc, const char **argv)
 		}
 		argv++; argc--;
 	}
-	if (combine_merges) {
+	if (dense_combined_merges)
 		diff_options.output_format = DIFF_FORMAT_PATCH;
-	}
 
 	/* Find the directory, and set up the pathspec */
 	pathspec = get_pathspec(prefix, argv + 1);
@@ -166,7 +165,8 @@ int main(int argc, const char **argv)
 			if (combine_merges && num_compare_stages == 2) {
 				show_combined_diff(&combine.p, 2,
 						   dense_combined_merges,
-						   NULL);
+						   NULL,
+						   &diff_options);
 				free(combine.p.path);
 				continue;
 			}