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 --git a/log-tree.c b/log-tree.c
index 1927f91..3fdc0fc 100644
--- a/log-tree.c
+++ b/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;