When fetch() is called and there is an existing file with the wrong size that

When fetch() is called and there is an existing file with the wrong size that
is smaller than PORTAGE_FETCH_RESUME_MIN_SIZE, rename the file instead of
deleting it. This is more friendly for cases in which the user is attempting
to regenerate the digest but has not used `ebuild --force foo.ebuild manifest`
like they're supposed to. Thanks to Patrick Lauer <patrick@g.o> for reporting.

svn path=/main/trunk/; revision=12739