diff --git a/diff-merges.c b/diff-merges.c
index 6d48ac5ab9..ee91ed0db4 100644
--- a/diff-merges.c
+++ b/diff-merges.c
@@ -10,6 +10,7 @@ static void suppress(struct rev_info *revs)
 	revs->dense_combined_merges = 0;
 	revs->combined_all_paths = 0;
 	revs->combined_imply_patch = 0;
+	revs->merges_need_diff = 0;
 }
 
 static void set_separate(struct rev_info *revs)
@@ -51,9 +52,13 @@ static void set_dense_combined(struct rev_info *revs)
 
 static void set_diff_merges(struct rev_info *revs, const char *optarg)
 {
-	if (!strcmp(optarg, "off") || !strcmp(optarg, "none"))
+	if (!strcmp(optarg, "off") || !strcmp(optarg, "none")) {
 		suppress(revs);
-	else if (!strcmp(optarg, "first-parent"))
+		/* Return early to leave revs->merges_need_diff unset */
+		return;
+	}
+
+	if (!strcmp(optarg, "first-parent"))
 		set_first_parent(revs);
 	else if (!strcmp(optarg, "separate"))
 		set_separate(revs);
@@ -63,6 +68,9 @@ static void set_diff_merges(struct rev_info *revs, const char *optarg)
 		set_dense_combined(revs);
 	else
 		die(_("unknown value for --diff-merges: %s"), optarg);
+
+	/* The flag is cleared by set_xxx() functions, so don't move this up */
+	revs->merges_need_diff = 1;
 }
 
 /*
@@ -129,10 +137,9 @@ void diff_merges_setup_revs(struct rev_info *revs)
 		revs->first_parent_merges = 0;
 	if (revs->combined_all_paths && !revs->combine_merges)
 		die("--combined-all-paths makes no sense without -c or --cc");
-	if (revs->combine_merges)
+	if (revs->combined_imply_patch)
 		revs->diff = 1;
-	if (revs->combined_imply_patch) {
-		/* Turn --cc/-c into -p --cc/-c when -p was not given */
+	if (revs->combined_imply_patch || revs->merges_need_diff) {
 		if (!revs->diffopt.output_format)
 			revs->diffopt.output_format = DIFF_FORMAT_PATCH;
 	}
diff --git a/log-tree.c b/log-tree.c
index f9385b1dae..9f50a81e53 100644
--- a/log-tree.c
+++ b/log-tree.c
@@ -899,15 +899,21 @@ static int log_tree_diff(struct rev_info *opt, struct commit *commit, struct log
 	int showed_log;
 	struct commit_list *parents;
 	struct object_id *oid;
+	int is_merge;
+	int all_need_diff = opt->diff || opt->diffopt.flags.exit_with_status;
 
-	if (!opt->diff && !opt->diffopt.flags.exit_with_status)
+	if (!all_need_diff && !opt->merges_need_diff)
 		return 0;
 
 	parse_commit_or_die(commit);
 	oid = get_commit_tree_oid(commit);
 
-	/* Root commit? */
 	parents = get_saved_parents(opt, commit);
+	is_merge = parents && parents->next;
+	if (!is_merge && !all_need_diff)
+		return 0;
+
+	/* Root commit? */
 	if (!parents) {
 		if (opt->show_root_diff) {
 			diff_root_tree_oid(oid, "", &opt->diffopt);
@@ -916,8 +922,7 @@ static int log_tree_diff(struct rev_info *opt, struct commit *commit, struct log
 		return !opt->loginfo;
 	}
 
-	/* More than one parent? */
-	if (parents->next) {
+	if (is_merge) {
 		if (opt->combine_merges)
 			return do_diff_combined(opt, commit);
 		if (opt->separate_merges) {
diff --git a/revision.h b/revision.h
index bfbae526ad..494d861424 100644
--- a/revision.h
+++ b/revision.h
@@ -194,6 +194,7 @@ struct rev_info {
 			always_show_header:1,
 			/* Diff-merge flags */
 			explicit_diff_merges: 1,
+			merges_need_diff: 1,
 			separate_merges: 1,
 			combine_merges:1,
 			combined_all_paths:1,