viewdiff: linkify diffstats for non-format-patch emails

viewdiff: linkify diffstats for non-format-patch emails

Some folks unfortunately use "git diff --stat -p" to generate
patches.  These messages lack the /^---$/ line and causing
diffstats to not get linkified properly.  We now treat the
/^---$/ as optional and rely on the presence of file lines with
/ \| / proceeding a /\d+ files? changed,/ line.