Make manifest mode with --force option cause existing digests to be replaced

Make manifest mode with --force option cause existing digests to be replaced
for any files that exist in ${DISTDIR}. This provides an alternative to
ebuild --force manifest for updating existing distfiles digests. Digests
are assumed to be corect for files that do not exist in ${DISTDIR} since
the user could simply remove the whole Manifest if they wanted to regenerate
digests for all files (and it's safer to regenerate as few as possible
because it's less probably that a valid digest will get replaced by an
invalid one).

svn path=/main/trunk/; revision=12802