Accidentally, this change also fixs a potential bug
if `XBPS_DIGEST_CMD` is changed to something other than
its current value: `xbps-uhelper digest'.
Because the old code compares:
- non-stripped old digest; and
- stripped new digest
We don't specify any specific digest function in this function,
and the old variables' name doesn't reflect the fact that they're
a pair of old and new digest built by the same hash function.
