hgweb: fix filediff base calculation Previously, we were finding the most recent version of a file in a changeset and comparing it against its first file parent. This was wrong on three counts: - it would show a diff in revisions where there was no change to a file - it would show a diff when only the exec bit changed - it would potentially compare against a much older changeset, which could be very expensive if git-style rename detection was enabled This compares the file in the current context with that context's parent, which may result in an empty diff when looking at a file not touched by the current changeset.