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 ...