Introduce get_octopus_merge_bases() in commit.c

This is like get_merge_bases() but it works for multiple heads, like
show-branch --merge-base.

Signed-off-by: Miklos Vajna <vmiklos@frugalware.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

commit.c

@@ -600,6 +600,33 @@ static struct commit_list *merge_bases(struct commit *one, struct commit *two)
 	return result;
 }
 
+struct commit_list *get_octopus_merge_bases(struct commit_list *in)
+{
+	struct commit_list *i, *j, *k, *ret = NULL;
+	struct commit_list **pptr = &ret;
+
+	for (i = in; i; i = i->next) {
+		if (!ret)
+			pptr = &commit_list_insert(i->item, pptr)->next;
+		else {
+			struct commit_list *new = NULL, *end = NULL;
+
+			for (j = ret; j; j = j->next) {
+				struct commit_list *bases;
+				bases = get_merge_bases(i->item, j->item, 1);
+				if (!new)
+					new = bases;
+				else
+					end->next = bases;
+				for (k = bases; k; k = k->next)
+					end = k;
+			}
+			ret = new;
+		}
+	}
+	return ret;
+}
+
 struct commit_list *get_merge_bases(struct commit *one,
 					struct commit *two, int cleanup)
 {

commit.h

@@ -121,6 +121,7 @@ int read_graft_file(const char *graft_file);
 struct commit_graft *lookup_commit_graft(const unsigned char *sha1);
 
 extern struct commit_list *get_merge_bases(struct commit *rev1, struct commit *rev2, int cleanup);
+extern struct commit_list *get_octopus_merge_bases(struct commit_list *in);
 
 extern int register_shallow(const unsigned char *sha1);
 extern int unregister_shallow(const unsigned char *sha1);