Bug #250166 - To avoid accidental regeneration of digests with the incorrect

Bug #250166 - To avoid accidental regeneration of digests with the incorrect
files (such as partially downloaded files), trigger the fetch code if the file
exists and it's size doesn't match the current manifest entry. If there really
is a legitimate reason for the digest to change, `ebuild --force digest` can
be used to avoid triggering this code (or else the old digests can be manually
removed from the Manifest).

svn path=/main/trunk/; revision=12186