Added --always-snapshot

The --always-snapshot path/to/file parameter can be repeated
any number of times, and must specify the relative path to each file
(not directory) that results corrupted after running --verify.
These files will be completely transferred again, even if they have
not changed since the last backup.

For instance:

CORRUPTED_MSG="Warning:\ Computed\ SHA1\ digest\ of\ "
CORRUPTED_ARGS=$(./rdiff-backup --verify user@host::/path/to/backup 2>&1 >/dev/null | grep "^$CORRUPTED_MSG" | sed "s/$CORRUPTED_MSG/--always-snapshot\ /g")
echo $CORRUPTED_ARGS