hgweb: don't point graph links at tip hash where it doesn't make sense Some pages, e.g. bookmarks, help and summary don't have a meaningful revision context: they always either show information about tip or about the whole repo (and not about any specific changeset). And error pages can just show hgweb error messages, not related to any repo or changeset. When monoblue style was added in 91b0ada2d94b, however, all graph links had tried to point at some hash, and on such pages as described above it didn't make sense. On error pages '{node|short}' is empty string anyway. Of course, it worked, but seeing such pages without revision context provide links with hashes is a bit confusing (unless you keep current tip hash in your head at all times) and wasn't consistent with other template styles, other pages in monoblue and even other links on the same page. Let's just link to '/graph', which is equal to '/graph/tip'.