linkrev-filelog: handle filtered linkrev with no visible children (issue4307) If the file revision with a filtered linkrev does not have any (unfiltered) children, we cannot use it to bound the search for another introduction. Instead, we have to look at the file revision used by each head changeset. If one of them uses this file revision, we know there is another occurrence and we have a starting point. See inline comments for details. Adding some kind of permanent reference of all the introductions of a file revision instead of just the first one would be much better. But this is more difficult. I hope to take that into account in the next repository format.