commit_list_insert: return the new commit list entry

This is useful for when we want to insert the next one after
this new one, for example.
diff --git a/commit.h b/commit.h
index f7a2cb8..edfdc12 100644
--- a/commit.h
+++ b/commit.h
@@ -26,7 +26,7 @@
 
 int parse_commit(struct commit *item);
 
-void commit_list_insert(struct commit *item, struct commit_list **list_p);
+struct commit_list * commit_list_insert(struct commit *item, struct commit_list **list_p);
 
 void free_commit_list(struct commit_list *list);