git-log --first-parent: show only the first parent log

If your development history does not have fast-forward merges,
i.e. the "first parent" of commits in your history are special
than other parents, this option gives a better overview of the
evolution of a particular branch.
Signed-off-by: NJunio C Hamano <junkio@cox.net>

revision.c

@@ -350,6 +350,7 @@ static void add_parents_to_list(struct rev_info *revs, struct commit *commit, st
 {
 	struct commit_list *parent = commit->parents;
 	unsigned left_flag;
+	int add, rest;
 
 	if (commit->object.flags & ADDED)
 		return;
@@ -395,18 +396,19 @@ static void add_parents_to_list(struct rev_info *revs, struct commit *commit, st
 		return;
 
 	left_flag = (commit->object.flags & SYMMETRIC_LEFT);
-	parent = commit->parents;
-	while (parent) {
+
+	rest = !revs->first_parent_only;
+	for (parent = commit->parents, add = 1; parent; add = rest) {
 		struct commit *p = parent->item;
 
 		parent = parent->next;
-
 		parse_commit(p);
 		p->object.flags |= left_flag;
 		if (p->object.flags & SEEN)
 			continue;
 		p->object.flags |= SEEN;
-		insert_by_date(p, list);
+		if (add)
+			insert_by_date(p, list);
 	}
 }
 
@@ -836,6 +838,10 @@ int setup_revisions(int argc, const char **argv, struct rev_info *revs, const ch
 				handle_all(revs, flags);
 				continue;
 			}
+			if (!strcmp(arg, "--first-parent")) {
+				revs->first_parent_only = 1;
+				continue;
+			}
 			if (!strcmp(arg, "--reflog")) {
 				handle_reflog(revs, flags);
 				continue;

revision.h

@@ -46,7 +46,8 @@ struct rev_info {
 			boundary:2,
 			left_right:1,
 			parents:1,
-			reverse:1;
+			reverse:1,
+			first_parent_only:1;
 
 	/* Diff flags */
 	unsigned int	diff:1,