diff-merges: introduce revs->first_parent_merges flag

This new field allows us to separate format of diff for merges from
'first_parent_only' flag which primary purpose is limiting history
traversal.

This change further localizes diff format selection logic into the
diff-merges.c file.

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

diff-merges.c

@@ -73,6 +73,8 @@ void diff_merges_default_to_first_parent(struct rev_info *revs)
 {
 	if (revs->ignore_merges < 0)		/* No -m */
 		revs->ignore_merges = 0;
+	if (!revs->combine_merges)		/* No -c/--cc" */
+		revs->first_parent_merges = 1;
 }
 
 void diff_merges_default_to_dense_combined(struct rev_info *revs)

log-tree.c

@@ -922,7 +922,7 @@ static int log_tree_diff(struct rev_info *opt, struct commit *commit, struct log
 			return 0;
 		else if (opt->combine_merges)
 			return do_diff_combined(opt, commit);
-		else if (!opt->first_parent_only) {
+		else if (!opt->first_parent_merges) {
 			/* If we show multiple diffs, show the parent info */
 			log->parent = parents->item;
 		}
@@ -941,7 +941,7 @@ static int log_tree_diff(struct rev_info *opt, struct commit *commit, struct log
 
 		/* Set up the log info for the next parent, if any.. */
 		parents = parents->next;
-		if (!parents || opt->first_parent_only)
+		if (!parents || opt->first_parent_merges)
 			break;
 		log->parent = parents->item;
 		opt->loginfo = log;

revision.h

@@ -194,6 +194,7 @@ struct rev_info {
 			combine_merges:1,
 			combined_all_paths:1,
 			dense_combined_merges:1,
+			first_parent_merges:1,
 			always_show_header:1;
 	int             ignore_merges:2;