log: include unmodified-in-merge files in log diff/stat (issue2383) f786fc4b8764 assumed that walkchangerevs called prep with all relevant matched filenames, but actually it only contains the names of files changed in the relevant changeset. That meant that log diff/stat of merges missed the diff for files only changed in the other branch. This is a minimal fix for making sure we only use fns when we are following and thus will have problems with merges anyway ...